JP7201136B1 - Steel sheet pile and its manufacturing method - Google Patents

Abstract

To stably provide high-strength and high-toughness steel sheet piles with high productivity. The steel sheet pile has a predetermined chemical composition, a microstructure having a ferrite area fraction of 70% or more and an island martensite area fraction of 1.0% or less, and a ferrite area fraction of 70% or more and Total area ratio of V carbonitride, Nb carbonitride, and V and Nb composite carbonitride having an area ratio of island martensite of 1.0% or less and a grain size of 10 nm or less is 2.6% and a microstructure in which the area ratio of V carbonitride, Nb carbonitride, and composite precipitates thereof having a predetermined grain size d (nm) is 0.3% or more, and the ferrite The average particle size is 15 μm or less and the maximum particle size is 40 μm or less.

Description

TECHNICAL FIELD The present invention relates to a steel sheet pile applied to permanent or temporary structures in the fields of civil engineering and construction, and a method for manufacturing the same.

Steel sheet piles are required to have high strength and toughness because they are subjected to high loads when used for quays and earth retaining walls. For example, a steel sheet pile having a yield strength (hereinafter referred to as YP) of 290 MPa or more, and further 390 MPa or more is used. Steel sheet piles having a strength of YP440 MPa or more may be essential under even more severe environments.

Addition of alloying elements and rolling in the non-recrystallization temperature range of austenite are common methods for manufacturing high-strength and high-toughness steel products. However, in the production of steel sheet piles with complicated shapes, from the standpoint of formability, rolling and forming at high temperatures with lower deformation resistance are preferred, and addition of alloys that can increase deformation resistance is restricted. be.

Although the JIS standard (SYW) for steel sheet piles stipulates the Charpy absorbed energy at 0°C, steel sheet piles are used even in environments below 0°C, such as during the cold season in Japan. It is expected that steel sheet piles will be required.

Under the above background, research and development of high-strength and high-toughness steel sheet piles are being conducted.
That is, Patent Document 1 proposes a steel sheet pile having a YP of 440 MPa or more and high toughness by adding more than 0.05% of Nb to the composition.

In Patent Document 2, the component composition is such that 0.030% or more of Nb is added together with V, and the rolling reduction at 1000° C. or less is controlled to reduce the average grain size of ferrite, the area ratio of island-shaped martensite, and the precipitates. Proposals have been made for steel sheet piles having a YP of 440 MPa or more and high toughness by optimizing the number density.

Patent Document 3 proposes a steel sheet pile having a chemical composition in which Nb and B are added to a low carbon steel containing 0.005 to 0.030% C, and which has high strength, high toughness, and excellent underwater weldability. .

In addition, in Patent Document 4, a component composition in which one or two types of V or Nb are added is used, the reduction ratio in the non-recrystallization temperature range is controlled at 900 ° C. or less, and accelerated cooling is performed after the completion of rolling to achieve high toughness. Steel sheet piles have been proposed.

On the other hand, Patent Document 5 proposes a steel sheet pile having a YP of 340 MPa or more and high toughness by limiting Nb in the inevitable impurities to 0.005% or less.

Patent Documents 6 and 7 propose a steel sheet pile having a YP of 440 MPa or more and a vTrs of −10° C. or less by water-cooling a predetermined portion during or after hot rolling.

JP 2018-83963 A JP 2018-90845 A JP-A-2000-17378 JP 2006-249513 A JP-A-2002-294392 JP 2007-332414 A JP 2008-221318 A

In the techniques described in Patent Documents 1 and 2, steel sheet piles having high strength and high toughness are obtained by adding 0.030% or more of Nb to the composition. It tends to increase the deformation resistance during hot rolling, and it is necessary to strictly control the shape during hot rolling.

In the technique described in Patent Document 3, the amount of C is as low as 0.005 to 0.030%, so the decarburization process during steel smelting takes a long time, and the productivity in the refining process is low. There's a problem.

In addition, in the technique described in Patent Document 4, in order to obtain a steel sheet pile with high toughness, the rolling reduction in the non-recrystallization temperature range must be 20% or more, and the shape control during hot rolling must be strict. In addition, since accelerated cooling of a predetermined portion is required after completion of rolling, there is a problem that shape change such as bending and warping is unavoidable.

On the other hand, the technique described in Patent Document 5 promotes complete recrystallization of austenite by limiting the rolling temperature and the rolling reduction of the final pass, and by obtaining a uniform structure, a YP of 340 MPa or more and high toughness. However, the YP is less than 440 MPa, and further improvement of YP is required.

In the technique described in Patent Document 6 or 7, in order to obtain a steel sheet pile having a YP of 440 MPa or more and a vTrs of −10° C. or less, water cooling of a predetermined portion is essential, so shape changes such as bending and warping are unavoidable. The problem is that

An object of the present invention is to solve the above-mentioned problems, and to provide steel sheet piles with high strength and high toughness stably and with high productivity. Here, high strength means that YP is 440 MPa or more, and high toughness means that the fracture surface transition temperature (hereinafter referred to as vTrs) at which the ductile fracture surface ratio becomes 50% is -10°C or less.

Reducing the number of fracture units by refining grains is an effective means of ensuring high toughness. As one means for refining crystal grains, there is a method for refining crystal grains by using precipitates present in prior austenite grains or at grain boundaries as ferrite generation nuclei. The present inventors investigated the mechanism of grain refinement by precipitates, and found that the precipitates must have a certain grain size or more in order for the precipitates to contribute to the refinement of ferrite grains. have understood. It was also found that the critical grain size tends to decrease as the ferrite transformation initiation temperature decreases.

In order to secure high strength, precipitation dispersion strengthening by precipitates is one of the effective means. become conspicuous.

In order to secure high toughness, precipitates having a certain grain size or more are necessary, while fine precipitates are effective in securing high strength. These two are in an antinomic relationship, and conventionally it has been difficult to achieve both high strength and high toughness by utilizing precipitates. One way to solve this problem is to lower the ferrite transformation starting temperature by accelerated cooling, but this is not preferable because it may cause bending or warping. Further, positive rolling in the non-recrystallization temperature range is one of the effective means, but it is necessary to strictly control the shape.

By the way, it is effective to use a component composition containing V in order to refine crystal grains by precipitates. That is, V precipitates in austenite as V carbonitride in a relatively high temperature range and has a coherent interface with ferrite, so it is characterized in that it greatly contributes to the formation of ferrite nuclei. Moreover, in order to ensure high strength, it is effective to use a component composition containing Nb. That is, since Nb finely precipitates in austenite as Nb carbonitrides with grain sizes on the order of several nanometers mainly due to strain induction, dispersion strengthening by the precipitates is expected.

Based on these facts, the present inventors made a component composition in which V and Nb are added in combination, and investigated the component composition ratio of V and Nb in order to control precipitates. As a result, in order to achieve both refinement of crystal grains by using V carbonitrides or composite carbonitrides of V and Nb as ferrite nuclei and dispersion strengthening by precipitates, the components of V and Nb It was found that there is an appropriate range for the composition ratio.

In addition, just optimizing the component composition ratio of V and Nb is not enough, and when we examined rolling conditions that can ensure high productivity and high strength and high toughness, we found that It has been found that effective rolling is effective. That is, it is important to precipitate Nb carbonitride precipitates or composite carbonitrides of V and Nb by strain induction. This makes it possible to relax the restrictions on rolling conditions over a wide temperature range as in the past and the reduction rate (hereinafter also referred to as the CR rate) in the non-recrystallization temperature range of austenite, and also affects the occurrence of bending and warping. There is no need to perform accelerated cooling that affects the

In order to effectively improve the strength and toughness of V precipitates, Nb precipitates, and composite precipitates thereof, the present inventors have found that fine particle size precipitates that can contribute to an increase in strength It has been found that a certain or more area ratio is required for precipitates with coarse grains that can contribute to the formation of ferrite nuclei.

From the above, the present inventors have found that due to dispersion strengthening and uniform refinement of the structure by V carbonitride, Nb carbonitride, or composite precipitates thereof, high strength of YP440 MPa or more and vTrs of -10 ° C. or less We have found a way to provide a steel sheet pile with a high toughness. The gist of the present invention is as follows.

1. in % by mass,
C: 0.05 to 0.18%,
Si: 0.05 to 0.55%,
Mn: 1.00-1.65%,
sol. Al: 0.080% or less,
V: 0.005 to 0.250%,
Nb: 0.005% or more and less than 0.030% and N: 0.0010 to 0.0060%
in a range that satisfies the following formula (1), the balance being Fe and unavoidable impurities, and P, S and B as the unavoidable impurities are: P: 0.025% or less, S: 0.020 % or less and B: 0.0003% or less,
V carbonitride, Nb carbonitride, and V and Nb composite carbonitride having a ferrite area fraction of 70% or more, an island martensite area fraction of 1.0% or less, and a grain size of 10 nm or less is 2.6% or more, and the total area ratio of V carbonitride, Nb carbonitride, and V and Nb composite precipitates having a grain size d (nm) that satisfies the following formula (2) is a microstructure that is greater than or equal to 0.30%;
The ferrite has an average grain size of 15 μm or less and a maximum grain size of 40 μm or less,
A steel sheet pile having a yield strength of 440 MPa or more and a vTrs of -10°C or less.
Note -0.010 ≤ [% Nb] - 0.1 [% V] ≤ 0.020 (1)
Here, [%V] and [%Nb] are the contents (% by mass) of V and Nb in the steel, respectively.
d≧5[(Ae ₃ −Ar ₃ )/Ae ₃ ] ^−0.63 (2)
Here, Ae ₃ : Ferrite transformation start temperature (°C) during equilibrium transformation
Ar ₃ : ferrite transformation start temperature during cooling (°C)

Ａｒ_３＝９１０－３１０[％Ｃ]－８０[％Ｍｎ]－２０[％Ｃｕ]－１５[％Ｃｒ]－５５[％Ｎｉ]－８０[％Ｍо] ・・・（４）
ここで、[％Ｃ]、[％Ｓｉ]、[％Ｍｎ]、[％Ｃｕ]、[％Ｃｒ]、[％Ｎｉ]および[％Ｍо]はそれぞれ、鋼中のＣ、Ｓｉ、Ｍｎ、Ｃｕ、Ｃｒ、ＮｉおよびＭｏの含有量（質量％）である。The above Ae ₃ and Ar ₃ are obtained according to the following equations (3) and (4), respectively.

Ar ₃ =910-310[%C]-80[%Mn]-20[%Cu]-15[%Cr]-55[%Ni]-80[%Mo] (4)
Here, [%C], [%Si], [%Mn], [%Cu], [%Cr], [%Ni] and [%Mo] are respectively C, Si, Mn, Cu , Cr, Ni and Mo contents (% by mass).

2. The component composition is further mass %,
Cu: 0.50% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Mo: 0.30% or less,
Ca: 0.0050% or less,
2. The steel sheet pile according to 1 above, containing one or more of Ti: 0.025% or less and REM: 0.005% or less.

3. A steel material having the chemical composition described in 1 or 2 above is heated to 1200° C. to 1350° C. and subjected to hot rolling including rough rolling, intermediate rolling and finish rolling at a cumulative rolling reduction of 20% at 900° C. to 1000° C. As described above, the rolling reduction in the austenite non-recrystallization temperature range is 10% or more and less than 20%, and the end temperature of the intermediate rolling is 650 ° C. to 900 ° C., the yield strength is 440 MPa or more and vTrs is -10 ° C. or less. A method for manufacturing steel sheet piles.

INDUSTRIAL APPLICABILITY According to the present invention, high-strength and high-toughness steel sheet piles having a YP of 440 MPa or more and a vTrs of -10° C. or less can be stably provided with high productivity, which is very useful industrially.

It is a figure which shows the cross-sectional shape of a steel sheet pile. FIG. 3 is a diagram showing a typical caliber in a hot rolling process of a hat-shaped steel sheet pile. 4 is a graph showing the relationship between the area ratio of precipitates satisfying formula (2) and vTrs. 4 is a graph showing the relationship between the area ratio of precipitates having a size of 10 nm or less and yield strength.

As a form of the steel sheet pile, the hat-shaped steel sheet pile 1 shown in FIG. 1(a) is a typical example, and there is also the straight steel sheet pile 9 shown in FIG. The final shape is given through a mold. This form and manufacturing procedure will be described later in detail.
<Steel sheet pile>
The chemical composition and microstructure of the steel sheet pile of the present invention will be described in detail below.
[Composition]
First, the reason for limiting the chemical composition of the steel sheet pile of the present invention will be described. In the following description, "%" of the content of each element means "% by mass" unless otherwise specified.
In this specification, V carbonitride is "V precipitate" or "V (C, N)", Nb carbonitride is "Nb precipitate" or "Nb (C, N)", V and Nb composite Carbonitrides are also referred to as "(their) composite precipitates" or "(V,Nb)(C,N)".

C: 0.05-0.18%
C combines with V, Nb and N in steel, precipitates as carbonitrides such as V(C,N), Nb(C,N) or (V,Nb)(C,N), and increases the strength of the base material. Ni is an essential element for stably securing toughness and must be added in an amount of 0.05% or more. On the other hand, if it exceeds 0.18%, bainite containing island-shaped martensite is generated, and the increase in island-shaped martensite significantly reduces the toughness, and excessive precipitates reduce the toughness. Therefore, in the present invention, the C content is made 0.05 to 0.18%. Furthermore, the C content is preferably 0.10% or more. Also, the C content is preferably 0.16% or less.

Si: 0.05-0.55%
Si is an element that enhances the strength of the base material by solid solution strengthening, and should be contained in an amount of 0.05% or more. On the other hand, if the Si content is excessive, it promotes the formation of island-shaped martensite that reduces the toughness, so the Si content is made 0.55% or less. Therefore, the Si content should be 0.05 to 0.55%. Furthermore, the Si content is preferably 0.10% or more. Also, the Si content is preferably 0.50% or less.

Mn: 1.00-1.65%
Like Si, Mn is a relatively inexpensive element that has the effect of increasing the strength of steel, so it is an element necessary for increasing the strength. However, when the Mn content is less than 1.00%, the effect is reduced. On the other hand, if it exceeds 1.65%, it promotes the formation of upper bainite including island-shaped martensite and greatly impairs the toughness. Therefore, the Mn content is set to 1.00 to 1.65%. Furthermore, the Mn content is preferably 1.10% or more. Also, the Mn content is preferably 1.60% or less.

sol. Al: 0.080% or less Al is an element added as a deoxidizing agent. However, the effect of Al as a deoxidizing agent is sol. Since it is saturated when it exceeds 0.080% as Al, sol. Al is set to 0.080% or less. Although the lower limit is not particularly limited, it is preferably 0.001% or more for deoxidation. More preferably, it is 0.003% or more. Moreover, it is preferable that it is 0.060% or less.

V: 0.005-0.250%
V precipitates in austenite as V (C, N) or (V, Nb) (C, N) during rolling or cooling, contributes as a ferrite nucleation site, and has the effect of refining crystal grains. is an important element possessing Moreover, V has a role of increasing the strength of the base material by dispersion strengthening as a precipitate, and is an essential element for ensuring strength and toughness. In order to obtain the above effects, the V content must be 0.005% or more. On the other hand, if the V content exceeds 0.250%, it promotes precipitation embrittlement and greatly impairs the toughness of the base material. Therefore, the V content is set to 0.005 to 0.250%. Furthermore, the V content is preferably 0.075% or more. More preferably, it exceeds 0.080%. Also, the V content is preferably 0.200% or less.

Nb: 0.005% or more and less than 0.030% Nb is mainly strain-induced precipitation during rolling to form Nb (C, N) or (V, Nb) (C, N) in austenite with a size on the order of several nm. It has the effect of suppressing recrystallization of austenite and refining crystal grains. Moreover, Nb has a role of increasing the strength of the base material by dispersion strengthening as a precipitate, and is an essential element for ensuring strength and toughness. To obtain such effects, the Nb content must be 0.005% or more. Preferably, it is 0.010% or more. On the other hand, Nb increases hot deformation resistance and, if the content is 0.030% or more, precipitation embrittlement tends to reduce toughness, so the Nb content is set to less than 0.030%. Preferably, it is 0.025% or less.

N: 0.0010 to 0.0060%
N combines with V, Nb and C in steel, and forms V(C,N), Nb(C,N) or (V,Nb)(C,N) to improve base material strength and toughness. Ni is a useful element and should be contained in an amount of 0.0010% or more. However, if the N content exceeds 0.0060%, the toughness of the base material is greatly impaired due to precipitation embrittlement. Therefore, in the present invention, the N content is made 0.0010 to 0.0060%. Furthermore, the N content is preferably 0.0015% or more. Also, the N content is preferably 0.0055% or less.

Furthermore, in the present invention, in addition to each element satisfying the above range, for V and Nb, the contents (% by mass) of V and Nb are respectively [%V] and [%Nb], It is important to satisfy the relationship of the following formula (1).
−0.010≦[%Nb]−0.1[%V]≦0.020 (1)

The inventors evaluated the strength and toughness using various steel sheet piles having steel compositions within the above range. , N) or (V, Nb) (C, N) was found to be important. Specifically, when the value calculated by [%Nb]-0.1 [%V] is less than -0.010, that is, when V is excessive, V is V (C, N) or (V , Nb) (C, N) precipitates in large quantities at high temperatures and coarsens, and the strain-induced precipitation amount of fine Nb (C, N) or (V, Nb) (C, N) decreases. Therefore, it was found that the strength decreased.

That is, by controlling the value calculated by the above formula, which is a parameter based on the content of V and Nb, to −0.010 or more, V contributes as a ferrite nucleation site and uniformly refines the structure. As a result, it is possible to secure a sufficient size and amount of precipitates V (C, N) or (V, Nb) (C, N) that contribute to the improvement of toughness, and a sufficient amount that contributes to the increase in strength. Finely precipitated Nb(C,N) or (V,Nb)(C,N) can be secured. On the other hand, when the value [%Nb]-0.1 [%V] is more than 0.020, that is, when Nb is excessive, it becomes a ferrite nucleation site and contributes to the improvement of toughness. and the amount of precipitated V(C,N) or (V,Nb)(C,N) decreases, impairing the toughness of the base material. Therefore, in the present invention, the value calculated by [%Nb]-0.1[%V] is in the range of -0.010 to 0.020. The value calculated by the above formula is preferably -0.005 or more. Moreover, it is preferable that it is 0.015 or less.

In the chemical composition of the present invention, the balance other than the above elements is Fe and unavoidable impurities. Among the unavoidable impurities, the upper limit of the content of P, S, and B is set as shown below.

P: 0.025% or less P is present as an unavoidable impurity in steel, but if the P content is excessive, the toughness of the steel is lowered, so P is made 0.025% or less. The P content is preferably as small as possible, and may be 0%, but an excessive reduction in the P content leads to a decrease in productivity due to a prolonged refining process, so the P content is 0.005%. It is preferable to set it as above.

S: 0.020% or less S, like P, is contained in steel as an unavoidable impurity and also exists as A-based inclusions. If the S content is excessive, the amount of A-based inclusions increases excessively and the toughness of the steel deteriorates, so the S content is made 0.020% or less. The S content is preferably as small as possible, and may even be 0%. It is preferable to set it as above.

B: 0.0003% or less B is an element that segregates at grain boundaries in steel and has the effect of increasing grain boundary strength. When low-quality raw materials are used, the steel may contain more than 0.0003% of B. In this case, coarse grain boundary precipitates are formed, and the hardenability increases, which promotes the formation of island-shaped martensite and lowers the toughness. Furthermore, it is preferable to make it 0.0002% or less.

Although the above is the basic component composition in the present invention, one or more of the following elements may be contained as necessary.
Cu: 0.50% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Mo: 0.30% or less,
Ca: 0.0050% or less,
Ti: 0.025% or less and REM: 0.005% or less

Cu: 0.50% or less Cu is an element capable of further increasing the strength of steel through solid-solution strengthening. In order to obtain such effects, it is preferable that the Cu content is 0.01% or more. However, when the content exceeds 0.50%, Cu cracks are likely to occur. Therefore, when Cu is contained as a chemical composition of steel, the content is preferably 0.50% or less.

Ni: 0.50% or less Like Cu, Ni is an element that dissolves in the steel and increases the strength of the steel without degrading the ductility and toughness. In order to obtain such effects, it is preferable that the Ni content is 0.01% or more. In particular, it is preferable to suppress Cu cracking by adding Ni in combination with Cu. On the other hand, an excessive Ni content promotes the formation of island-shaped martensite, and Ni is an expensive element. From these points of view, the Ni content is set to 0.50% or less. is preferred.

Cr: 0.50% or less Cr is an element capable of further increasing the strength of steel through solid-solution strengthening. In order to obtain such effects, it is preferable that the Cr content is 0.01% or more. On the other hand, an excessive Cr content promotes the formation of island martensite, so the Cr content is preferably 0.50% or less.

Mo: 0.30% or less Mo is an element capable of further increasing the strength of steel through solid-solution strengthening. In order to obtain such effects, the Mo content is preferably 0.01% or more. On the other hand, an excessive Mo content promotes the formation of island-shaped martensite, so the Mo content is preferably 0.30% or less.

Ca: 0.0050% or less Ca combines with S and O to reduce MnS in the steel, thereby improving the toughness and ductility of the steel. In order to obtain such effects, it is preferable that the Ca content is 0.0005% or more. On the other hand, if the Ca content exceeds 0.0050%, the cleanliness tends to decrease and the toughness tends to decrease, so the Ca content is preferably 0.0050% or less.

Ti: 0.025% or less Ti precipitates as TiN in austenite and has the effect of refining crystal grains. In order to obtain such effects, it is preferable that the Ti content is 0.001% or more. On the other hand, if the Ti content is excessive, the precipitated TiN becomes coarse and the crystal grains become coarse, which tends to lower the toughness. Therefore, it is preferable to set the Ti content to 0.025% or less.

REM: 0.005% or less Like Ca, REM (rare earth element) combines with S and O to reduce MnS in the steel, thereby improving the toughness and ductility of the steel. In order to obtain such effects, it is preferable that the REM content is 0.001% or more. On the other hand, if the Ca content exceeds 0.005%, the cleanliness tends to decrease and the toughness tends to decrease, so the REM content is preferably 0.005% or less.

Next, the microstructure of the steel sheet pile in the present invention will be explained. In addition, in the present invention, the microstructure of the web portion of the steel sheet pile may be specified. This is because the web portion has the lowest working degree and coarse structure, making it the most difficult to secure strength and toughness. will fulfill.

The microstructure has an area fraction of ferrite of 70% or more, an area fraction of island martensite of 1.0% or less, an average grain size of ferrite of 15 μm or less, and a maximum grain size of 40 μm or less. is essential.

[Ferrite main organization]
The microstructure of the steel sheet pile is assumed to be a ferrite-based structure. A ferrite-based structure refers to a structure in which the area ratio of ferrite is 70% or more. If the ferrite area ratio is less than 70%, the hard phase may increase and the toughness may decrease. The upper limit of the area ratio of ferrite is preferably less than 90% from the viewpoint of ensuring strength. Although the second phase is not particularly limited, examples thereof include pearlite, bainite structure including island-shaped martensite, and martensite. However, the area ratio of island-shaped martensite will be limited later.

[The average grain size of ferrite is 15 µm or less and the maximum grain size is 40 µm or less]
In the microstructure of the steel sheet pile, ferrite has an average grain size of 15 μm or less and a maximum grain size of 40 μm or less. If the average grain size of ferrite is larger than 15 μm or the maximum grain size is larger than 40 μm, YP will decrease and it will be difficult to ensure toughness. In order to obtain even higher strength and toughness, it is desirable that the ferrite has an average grain size of 12 μm or less and a maximum grain size of 30 μm or less. The average grain size and maximum grain size of ferrite can be measured according to the measuring method described in Examples below.
Although the lower limit of the average grain size of ferrite is not particularly limited, it is preferably 5 μm or more, more preferably 10 μm or more, from the viewpoint of ensuring tensile strength. The lower limit of the maximum grain size of ferrite is not particularly limited, but is preferably 20 μm or more, more preferably 25 μm or more, from the viewpoint of securing tensile strength.

[Area ratio of island martensite is 1.0% or less]
In the microstructure of the steel sheet pile, the area ratio of island-shaped martensite is set to 1.0% or less. If the area ratio of island-shaped martensite is more than 1.0%, it becomes difficult to ensure toughness. In order to obtain higher toughness, it is desirable to set the area ratio of island martensite to 0.5% or less. The smaller the area ratio of island-shaped martensite, the better. The area ratio of island-shaped martensite can be measured according to the measuring method described in Examples below.

Furthermore, in order to obtain high-strength and high-toughness steel sheet piles, dispersion strengthening by precipitates and uniform refinement of the structure by precipitates are effective, so the grain size and area ratio of precipitates were also investigated. .
That is, steel materials having various chemical compositions within the chemical composition ranges described above according to the present invention were hot-rolled to form various steel sheet piles, and then air-cooled (cooling rate: 0.50 ° C./s). . A sample was taken from each steel sheet pile after air cooling, surface-treated by an electrolytic polishing method, and then observed with a transmission electron microscope (hereinafter referred to as TEM). The particle size and area ratio of the precipitate, which consisted of a substance, were measured. The grain size and area ratio of precipitates were measured according to the method described later.

For the various steel sheet piles described above, Charpy impact test specimens were taken from each steel sheet pile to determine the fracture surface transition temperature (vTrs) at a ductile fracture surface rate of 50%. In addition, vTrs was measured according to the method described in Examples below.

Ａｒ_３＝９１０－３１０[％Ｃ]－８０[％Ｍｎ]－２０[％Ｃｕ]－１５[％Ｃｒ]－５５[％Ｎｉ]－８０[％Ｍо] ・・・（４）
ここで、[％Ｃ]、[％Ｓｉ]、[％Ｍｎ]、[％Ｃｕ]、[％Ｃｒ]、[％Ｎｉ]および[％Ｍо]はそれぞれ、鋼中のＣ、Ｓｉ、Ｍｎ、Ｃｕ、Ｃｒ、ＮｉおよびＭｏの含有量（質量％）である。The measurement result of vTrs is shown in FIG. 3 as a relationship with the area ratio of the precipitates. Here, the precipitates can be arranged according to the following formula (2).
d≧5[(Ae ₃ −Ar ₃ )/Ae ₃ ] ^−0.63 (2)
Here, d, Ae ₃ and Ar ₃ are the grain size (nm) of the precipitate, the ferrite transformation start temperature (°C) during equilibrium transformation, and the ferrite transformation start temperature (°C) during air cooling, respectively. Ae ₃ and Ar ₃ are determined by the following equations (3) and (4), respectively.

Ar ₃ =910-310[%C]-80[%Mn]-20[%Cu]-15[%Cr]-55[%Ni]-80[%Mo] (4)
Here, [%C], [%Si], [%Mn], [%Cu], [%Cr], [%Ni] and [%Mo] are respectively C, Si, Mn, Cu , Cr, Ni and Mo contents (% by mass).

FIG. 3 shows the area ratio of V carbonitrides, Nb carbonitrides and their composite precipitates having a precipitate grain size that satisfies the formula (2) and the fracture surface transition temperature (° C.) (hereinafter “vTrs”). Show relationship. It can be seen that when the total area ratio of V carbonitride, Nb carbonitride and their composite precipitates having a precipitate grain size that satisfies formula (2) is 0.30% or more, vTrs-10 ° C. or less. . The total area ratio is preferably 0.35% or more.
Although the upper limit of the total area ratio is not particularly limited, it is preferably 1.00% or less from the viewpoint of suppressing excessive precipitation embrittlement.

Furthermore, for the various steel sheet piles described above, tensile test pieces were taken from each steel sheet pile to determine YP (0.2% yield strength). YP was measured according to the method described in Examples below. Regarding this measurement result, the relationship with the area ratio of V carbonitride, Nb carbonitride and their composite precipitates having a particle size of 10 nm or less, extracted from the measurement results of the particle size and area ratio of the precipitates described above. , as shown in FIG. Here, the reason for focusing on the area ratio of V carbonitrides, Nb carbonitrides and their composite precipitates having a grain size of 10 nm or less is that the Orowan stress related to precipitation strengthening is inversely proportional to the grain size of the precipitates. It is because it becomes an index when utilizing precipitation strengthening. From FIG. 4, it can be seen that the YP is 440 MPa or more when the total area ratio of V carbonitrides, Nb carbonitrides and their composite precipitates with grain sizes of 10 nm or less is 2.6% or more. The total area ratio is preferably 4.0% or more.
Although the upper limit of the total area ratio is not particularly limited, it is preferably 10.0% or less from the viewpoint of suppressing excessive precipitation embrittlement.

<Manufacturing method of steel sheet pile>
Next, the method for manufacturing the steel sheet pile of the present invention will be described.
A steel sheet pile is manufactured by heating a steel material such as a slab having the above compositional components in a heating furnace and then subjecting it 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 steel sheet piles. A hat-shaped steel sheet pile 1 includes a web 2, a pair of flanges 3 and 4 extending obliquely from both ends of the web 2, and extending parallel to the web 2 from opposite sides of the flanges 3 and 4. It has arms 5 and 6 that are present and claws 7 and 8 that are at the ends of arms 5 and 6 .

Taking the manufacture of this hat-shaped steel sheet pile as an example, after heating the steel material, the steel material is finally passed through a caliber as shown in FIG. 2 in each of rough rolling, intermediate rolling and finish rolling. Specifically, after the steel material is rolled a plurality of times in the initial rough rolling, it finally passes through the caliber 13 to form the rough shape of the steel sheet pile. In subsequent intermediate rolling, the web 2, the flanges 3 and 4, the arms 5 and 6, and the claws 7 and 8 are adjusted in thickness while finally passing through the groove 14. Furthermore, in the finish rolling, shape control including mainly claw bending forming is performed, and finally it passes through the caliber 15 to obtain the final product shape.

Steel sheet piles other than the hat-shaped steel sheet piles shown above, for example, steel sheet piles with different product shapes including web thickness and claw portions, such as the straight steel sheet pile 9 shown in FIG. There may be differences in the number of rolling passes and rolling temperature in hot rolling, but there is no fundamental difference in the production of rough rolling, intermediate rolling and finish rolling (including claw bending forming), all of which are Included in the manufacturing method of the invention. Here, in the straight steel sheet pile 9 shown in FIG.

After heating the steel material to 1200 ° C. to 1350 ° C., hot rolling is performed at a cumulative rolling reduction of 20% or more at 900 ° C. to 1000 ° C., and a rolling reduction in the non-recrystallized austenite region (hereinafter also referred to as CR rate ) is 10% or more and less than 20% and the finishing temperature of intermediate rolling is 650°C to 900°C.

[Heating temperature of steel material: 1200°C to 1350°C]
When performing hot rolling, it is necessary to heat the steel material to 1200°C to 1350°C. If the heating temperature is lower than 1200°C, the solid solution of V and Nb in the steel components becomes insufficient, and the precipitates become coarse, making it difficult to ensure strength, and the hot deformation resistance increases, resulting in an increase in rolling resistance. There is a risk that the roll will break. On the other hand, if the heating temperature exceeds 1350° C., the crystal grains become coarse, making it difficult to ensure toughness, and the heating time increases, resulting in a decrease in productivity. Therefore, the heating temperature of the steel material is set to 1200°C to 1350°C. Preferably, it is 1250° C. or higher.

[Cumulative rolling reduction at 900°C to 1000°C is 20% or more]
It is important that the cumulative rolling reduction at 900°C to 1000°C is 20% or more. By setting the reduction rate just above the non-recrystallization temperature range to 20% or more, Nb carbonitrides or composite precipitates of Nb and V of several nm order are precipitated in austenite due to strain induction, and YP is significantly improved. do. Preferably, it is 25% or more. The upper limit is not particularly limited, but is preferably 30% or less from the viewpoint of manufacturability.

[CR rate is 10% or more and less than 20%]
In addition to the above, it is important that the CR rate is 10% or more. When the CR rate is less than 10%, the grain size in the microstructure eventually becomes coarse, and the average grain size of ferrite becomes larger than 15 μm or the maximum grain size becomes larger than 40 μm, making it difficult to secure toughness. . Moreover, since the rolling load increases and it becomes necessary to strictly control the shape, the CR ratio is set to less than 20%. Preferably, it is 13% or more and 18% or less. Here, the CR rate can be adjusted by increasing or decreasing the roll gap during feeding.

[End temperature of intermediate rolling is 650°C to 900°C]
The end temperature of the intermediate rolling for forming the web and flanges is 650°C to 900°C. If the temperature exceeds 900°C, it becomes difficult to satisfy either of the above two rolling conditions, and finally the average grain size of ferrite in the microstructure becomes larger than 15 μm or the maximum grain size becomes larger than 40 μm, and toughness cannot be secured. Difficulties can arise. It is preferably 850° C. or less. On the other hand, if the temperature is lower than 650° C., the rolling load in the intermediate rolling increases, and the risk of roll breakage in the intermediate rolling mill increases. Preferably, it is 700°C or higher. As described above, intermediate rolling refers to rolling from rough rolling that gives the outline of the steel sheet pile to nail bending rolling, and mainly the portion that will become the web is rolled down in the thickness direction. It is about rolling.

The steel sheet pile of the present invention does not require accelerated cooling during rolling or after claw bending rolling for the purpose of improving strength and toughness. Accelerated cooling is not preferable in terms of production because it causes shape changes such as bending and warping. Therefore, air cooling is desirable after nail bending rolling. From the viewpoint of shape control during rolling, water that is unavoidably applied and cooling such as mist water in the cooling bed do not affect the characteristics of the steel sheet pile of the present invention.

By performing component adjustment, rolling and cooling according to the above conditions, steel sheet piles can have excellent mechanical properties such as high strength of YP 440 MPa or more and vTrs of −10° C. or less. It should be noted that the steel sheet piles targeted by the present invention include hat-shaped, U-shaped, combinations thereof, straight-line, etc. regardless of the cross-sectional shape, and the thickness of the web and the shape of the claw portion are not particularly limited. .

Hereinafter, the configuration and effects of the present invention will be described more specifically according to examples. It should be noted that the present invention is not limited by the following examples, and can be modified as appropriate within the scope compatible with the gist of the present invention, and any of these are included in the technical scope of the present invention. .

Using a continuous casting machine, prepare steel materials having the chemical 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. 2, arm portions 5 and 6 extending in a direction in which a pair of flanges 3 and 4 extending obliquely from both ends of the web 2 extend in parallel to the left and right of the web 2, and the arms 5 and 6 A hat-shaped steel sheet pile was manufactured having claws 7 and 8 at both ends.

Observation of the microstructure of the steel sheet pile, observation of precipitates, tensile test and toughness test were carried out on the obtained steel sheet pile. Each evaluation method will be described below.

<Observation of microstructure>
A test piece was taken from the 1/4 position of the web thickness of the steel sheet pile web and used for observation of the microstructure. Prior to observation, the test pieces collected here were surface-polished and corroded with nital. Then, using an optical microscope, the cross section in the thickness direction of the web was observed at a magnification of 100 to identify the type of structure. Processing for conversion into three gradations of black and gray was performed for discrimination, and the area ratio of each tissue was obtained. Also, the average grain size of ferrite is obtained by calculating the area of each crystal grain of ferrite in the field of view by image analysis using the same watershed algorithm. It was obtained as an average value. The maximum grain size of ferrite was the maximum value among the circle-equivalent diameters within the field of view. Furthermore, regarding the observation of martensite islands, the cementite was dissolved by subjecting the same test piece as above to two-step etching treatment of electrolytic corrosion and nital, and a scanning electron microscope (SEM) was used at a magnification of about 1000 times. 10 or more fields of view were observed at random, and the area ratio of martensite islands was obtained by the same image analysis as described above.

<Observation of deposits>
A sample is taken from the 1/4 position of the web thickness of the steel sheet pile web, surface treated by electrolytic polishing, and then observed with a transmission electron microscope (TEM) at 200,000 times for 30 fields of view, and a grain size of 1 nm or more precipitates. For objects, using a dark field method and a mass spectrometer (EDS), V carbonitrides, Nb carbonitrides and composite precipitates thereof are identified, each precipitate is counted, and the number of precipitates occupying the field of view A total area ratio was calculated. However, the precipitate was regarded as an ellipse, and the average of the major axis and the minor axis was used as the grain size.

<Tensile test>
A JIS No. 1A tensile test piece specified in JIS Z2241 is taken from the 1/4 position of the web thickness of the steel sheet pile web so that the tensile direction is the longitudinal direction, and a tensile test is performed in accordance with JIS Z2241 to yield. Points (YP) and tensile strength (TS) were obtained.

<Toughness test>
A 2 mm V-notch Charpy impact test piece (V notch depth 2 mm) specified in JIS Z2242 was taken from the web thickness 1/4 position of the steel sheet pile web, and a Charpy impact test was performed according to JIS Z2242. The impact test was conducted in a temperature range of -80 to 40°C, and the absorbed energy (vE ₀ ) at 0°C and the fracture surface transition temperature (vTrs) at a ductile fracture surface ratio of 50% were determined.

Tables 2-1 and 2-2 also show the results of the above survey. Test results (No. 1 to 17 in Table 2-1) of steel sheet piles produced by the manufacturing method according to the present invention using compatible steel satisfying the chemical composition according to the present invention are all the desired properties (yield strength YP: 440 MPa or more and a fracture surface transition temperature vTrs with a ductile fracture surface ratio of 50%: −10° C. or less) were satisfied.

On the other hand, the steel composition of the steel sheet pile does not satisfy the conditions of the present invention, or does not satisfy the conditions of the manufacturing method of the present invention, or does not satisfy any of the above. Nos. 18 to 38, 41) do not satisfy the required properties in either the yield strength or the fracture surface transition temperature (vTrs) at a ductile fracture surface ratio of 50%.

In addition, in all of the comparative examples (Nos. 39, 40, and 42 in Table 2-2) that do not satisfy the conditions of the production method of the present invention, the rolling load during intermediate rolling was excessive, and the load capacity of the rolls was exceeded. was discontinued.

1: Hat-shaped steel sheet pile 2: Web
3: Flange 4: Flange 5: Arm 6: Arm 7: Claw 8: Claw 9: Straight steel sheet pile 10: Web 11: Claw 12: Claw 13: Final rough rolling of hat-shaped steel sheet pile Pass caliber 14: Final pass caliber in intermediate rolling of hat-shaped steel sheet pile 15: Final pass caliber in finish rolling of hat-shaped steel sheet pile

Claims (3)

in % by mass,
C: 0.05 to 0.18%,
Si: 0.05 to 0.55%,
Mn: 1.00-1.65%,
sol. Al: 0.080% or less,
V: 0.005 to 0.250%,
Nb: 0.005% or more and less than 0.030% and N: 0.0010 to 0.0060%
in a range that satisfies the following formula (1), the balance being Fe and unavoidable impurities, and P, S and B as the unavoidable impurities are: P: 0.025% or less, S: 0.020 % or less and B: 0.0003% or less,
V carbonitride, Nb carbonitride, and V and Nb composite carbonitride having a ferrite area fraction of 70% or more, an island martensite area fraction of 1.0% or less, and a grain size of 10 nm or less is 2.6% or more, and the total area ratio of V carbonitride, Nb carbonitride, and V and Nb composite carbonitride having a grain size d (nm) that satisfies the following formula (2) is 0.30% or more, and
The ferrite has an average grain size of 15 μm or less and a maximum grain size of 40 μm or less,
A steel sheet pile having a yield strength of 440 MPa or more and a vTrs of -10°C or less.
−0.010≦[%Nb]−0.1[%V]≦0.020 (1)
Here, [%V] and [%Nb] are the contents (% by mass) of V and Nb in the steel, respectively.
d≧5[(Ae ₃ −Ar ₃ )/Ae ₃ ] ^−0.63 (2)
Here, Ae ₃ : Ferrite transformation start temperature (°C) during equilibrium transformation
Ar ₃ : ferrite transformation start temperature during cooling (°C) The component composition is further mass %,
Cu: 0.50% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Mo: 0.30% or less,
Ca: 0.0050% or less,
The steel sheet pile according to claim 1, containing one or more of Ti: 0.025% or less and REM: 0.005% or less. A steel material having the chemical composition according to claim 1 or 2 is heated to 1200 ° C. to 1350 ° C., and subjected to hot rolling including rough rolling, intermediate rolling and finish rolling at a cumulative rolling reduction of 20 at 900 ° C. to 1000 ° C. % or more, the rolling reduction in the non-recrystallization temperature range of austenite is 10% or more and less than 20%, and the end temperature of the intermediate rolling is 650 ° C. to 900 ° C. ,
The intermediate rolling refers to rolling that rolls down a portion to be a web in the thickness direction, and refers to rolling before nail bending rolling,
The method for manufacturing a steel sheet pile according to claim 1 or 2, wherein the yield strength is 440 MPa or more and vTrs is -10°C or less.

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