JP7323056B2 - Steel sheet pile and its manufacturing method - Google Patents

Description

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

Steel sheet piles are required to have 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, or 390 MPa or more is used. On the other hand, steel sheet piles having a strength of YP440 MPa or more may be essential in a more severe environment.
Addition of alloying elements and rolling in the non-recrystallized region of austenite are common methods for producing high-strength and high-toughness steel products. However, in the production of steel sheet piles having a complicated shape, from the viewpoint of formability, rolling and forming at high temperatures with less deformation resistance are aimed, and the addition of alloys that can increase deformation resistance is sometimes limited.

The JIS standard (SYW) for steel sheet piles defines the Charpy absorbed energy at 0°C. However, since 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 with even higher toughness will be required in the future.

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 Nb to the composition.

In addition, in Patent Document 2, by setting the component composition to which both Nb and V are added and controlling the rolling reduction at 1000 ° C. or less, the average grain size of ferrite, the area ratio of island-shaped martensite, and the number density of precipitates We are proposing a steel sheet pile with a YP of 440 MPa or more and high toughness by optimizing the

In Patent Document 3, as in Patent Document 2, both Nb and V are added to the component composition, the cumulative rolling reduction at 900 ° C. or less is 90% or more, and the average grain size of ferrite and the number density of precipitates are optimized. A steel sheet pile having a YP of 460 MPa or more and high toughness has been proposed.

On the other hand, Patent Document 4 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 5 and 6 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 2014-118629 A JP-A-2002-294392 JP 2007-332414 A JP 2008-221318 A

The techniques described in Patent Literatures 1, 2 and 3 provide steel sheet piles with high strength and high toughness by adding Nb to the composition. However, Nb tends to increase the deformation resistance during hot rolling regardless of whether it is in a solid solution or a precipitated state, and it is necessary to strictly control the shape during hot rolling. Further, in the techniques described in Patent Documents 2 and 3, the grain size of ferrite is specified as an average value. However, Nb suppresses the recrystallization of austenite, causing variations in the precipitation state, which may result in mixed grains with different diameters of ferrite grains. Therefore, it is also required to stably obtain high toughness.

On the other hand, in Patent Document 4, a steel sheet pile having a YP of 340 MPa or more and high toughness by promoting complete recrystallization of austenite by limiting the rolling temperature and the reduction rate of the final pass and obtaining a uniform structure. has been proposed. However, YP is less than 440 MPa, and further improvement of YP is required.

In Patent Documents 5 and 6, 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. Therefore, the problem is that shape changes such as bending and warping are unavoidable.

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 vTrs is -10° C. or less. In addition, vTrs is the fracture surface transition temperature (temperature at which the ductile fracture surface ratio is 50%) measured by a Charpy impact test conforming to JIS Z2242, and is hereinafter also referred to as the fracture surface transition temperature at a ductile fracture surface ratio of 50%. say.

In order to obtain steel sheet piles with high strength and excellent toughness, the present inventors made V an essential component without using Nb, and furthermore, not only temperature control during hot rolling and management of cumulative rolling reduction, but also Focusing on the average rolling reduction per pass in the high temperature range, we conducted extensive investigations. As a result, by optimizing the rolling conditions, the structure is uniformly refined, and by utilizing the dispersion strengthening by V precipitates, it has a high strength of YP440 MPa or more and a high toughness of vTrs of -10 ° C or less. I have found a way to provide steel sheet piles.

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: 0.050-0.300% and N: 0.0010-0.0060%
The balance is Fe and inevitable impurities, and P, S and B as the inevitable impurities are P: 0.025% or less, S: 0.020% or less and B: 0.0003% or less has a component composition of
The microstructure is a ferrite-based structure, the average grain size of ferrite is 15 μm or less and the maximum grain size is 40 μm or less, and the area ratio of island martensite in the microstructure is 1.0% or less,
A steel sheet pile having a yield strength of 440 MPa or more and a vTrs of -10°C or less.

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,
Nb: 0.005% 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. The microstructure is a ferrite-based structure, the average grain size of ferrite is 15 μm or less and the maximum grain size is 40 μm or less, the area ratio of island martensite in the microstructure is 1.0% or less, and the yield A method for manufacturing a steel sheet pile having a strength of 440 MPa or more and a vTrs of −10° C. or less,
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: 0.050-0.300% and N: 0.0010-0.0060%
The balance is Fe and inevitable impurities, and P, S and B as the inevitable impurities are P: 0.025% or less, S: 0.020% or less and B: 0.0003% or less A steel material having a chemical composition is heated to 1200 ° C. to 1350 ° C.,
The steel material is subjected to hot rolling, and in the hot rolling,
Average rolling reduction per pass at 900°C to 1150°C is 10% or more,
The cumulative rolling reduction at 800 ° C to 1150 ° C is 60% or more, and the end temperature of intermediate rolling is 650 ° C to 900 ° C.
A method for manufacturing steel sheet piles.
Here, the intermediate rolling refers to rolling from rough rolling to finish rolling, and in the intermediate rolling, the thickness is adjusted mainly by reducing the web portion in the thickness direction.

4. 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,
Nb: 0.005% or less,
3. The method for producing a steel sheet pile according to 3 above, containing one or more of Ti: 0.025% or less and REM: 0.005% or less.

INDUSTRIAL APPLICABILITY According to the present invention, a high-strength and high-toughness steel sheet pile 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.

[Composition]
First, the reason for limiting the chemical composition of the steel sheet pile according to one embodiment of the present invention will be described. In the following description, "%" for the content of each element means "% by mass" unless otherwise specified.

C: 0.05-0.18%
C is an essential element for stably securing the strength of the base material as V(C, N) by combining with V and N in the steel, and must be added in an amount of 0.05% or more. On the other hand, when the C content exceeds 0.18%, bainite containing island martensite is generated, and the increase in island martensite greatly reduces toughness. In addition, the precipitates become excessive and the toughness further decreases. Therefore, 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, an excessive Si content promotes the formation of martensite islands that reduce toughness. Therefore, the Si content is set to 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, and 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 the Mn content exceeds 1.65%, it promotes the formation of upper bainite containing island-shaped martensite, which 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. When it exceeds 0.080% as Al, it saturates. Therefore, sol. Al is contained at 0.080% or less. sol. Al is preferably contained at 0.060% or less. sol. The lower limit of Al is not particularly limited, but sol. It is preferable to contain Al at 0.001% or more. sol. Al is more preferably contained at 0.003% or more.

V: 0.050-0.300%
V is an important element that precipitates in austenite as V(C,N) during rolling or cooling, contributes as a ferrite nucleation site, and has the effect of refining crystal grains. 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 enhance the above effect, the V content should be 0.050% or more. On the other hand, if the V content exceeds 0.300%, it promotes precipitation embrittlement and greatly impairs the toughness of the base material. Therefore, the V content is set to 0.050 to 0.300%. Furthermore, the V content is preferably 0.075% or more. The V content is more preferably greater than 0.080%. Also, the V content is preferably 0.200% or less.

N: 0.0010 to 0.0060%
N combines with V and C in steel and is an element useful as V(C,N) for improving the strength of the base material, and should be contained in an amount of 0.0010% or more. However, if the N content exceeds 0.0060%, the formed carbonitrides become coarse and the toughness of the base material is greatly impaired. Therefore, the N content is set to 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.

The above is the basic chemical composition of the steel sheet pile according to one embodiment of the present invention, and if necessary, one or more of the following elements may be contained.
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 Cu content exceeds 0.50%, Cu cracks tend to occur. Therefore, when Cu is contained as the chemical composition of steel, the Cu content is preferably 0.50% or less.

Ni: 0.50% or less Ni, like Cu, is an element capable of increasing the strength of steel without deteriorating ductility and toughness by dissolving in steel. In order to obtain such effects, it is preferable that the Ni content is 0.01% or more. In particular, Ni is combined with Cu to suppress Cu cracking. Therefore, Ni is preferably added together with Cu. On the other hand, an excessive Ni content promotes the formation of martensite islands. Also, Ni is an expensive element. Therefore, the Ni content is preferably 0.50% or less.

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, excessive Cr content promotes the formation of martensite islands. Therefore, 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, excessive Mo content promotes the formation of martensite islands. Therefore, the Mo content is preferably 0.30% or less.

Ca: 0.0050% or less Ca combines with S and O to reduce MnS in steel. This makes it possible to improve 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, when the Ca content exceeds 0.0050%, the cleanliness tends to decrease and the toughness tends to decrease. Therefore, it is preferable to set the Ca content to 0.0050% or less.

Nb: 0.005% or less Nb precipitates in austenite as Nb(C,N) during rolling, and has the effect of suppressing recrystallization of austenite and refining crystal grains. In order to obtain such effects, it is preferable that the Nb content is 0.001% or more. On the other hand, Nb tends to increase the deformation resistance during hot rolling regardless of whether it is in a solid solution or precipitated state. In particular, as will be described later, it is advantageous to set the Nb content to 0.005% or less when the average rolling reduction per pass in the recrystallization temperature range is 10% or more. Therefore, when Nb is included in the chemical composition of the steel, the Nb content is preferably 0.005% 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 REM content exceeds 0.005%, the cleanliness tends to decrease and the toughness tends to decrease. Therefore, it is preferable to set the REM content to 0.005% or less.

In the chemical composition of the steel sheet pile according to one embodiment of the present invention, the balance other than the above elements is Fe and unavoidable impurities. If the content of an element related to the optional additive component is less than each preferred lower limit, the element is treated as an unavoidable impurity (included as an unavoidable impurity). In addition, the total amount of unavoidable impurities is an amount that is unavoidably mixed into steel by a general manufacturing method, for example, preferably 0.050% or less, more preferably 0.040% or less. However, 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 exists as an unavoidable impurity in steel. However, if the P content is excessive, the toughness of the steel is lowered, so the P content is made 0.025% or less. The P content is preferably as small as possible, and may even be 0%. Therefore, it is more preferable to set the P content to 0.005% or more.

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 inclusions increases excessively and the toughness of the steel decreases. Therefore, the S content is set to 0.020% or less. The lower the S content, the better, and it may even be 0%. Therefore, the S content is more preferably 0.002% or more.

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. Therefore, the B content is made 0.0003% or less. Furthermore, the B content is preferably 0.0002% or less. The B content is preferably as small as possible, and may even be 0%.

Next, the microstructure of the steel sheet pile according to one embodiment of the present invention will be described. Here, the microstructure of the steel sheet pile according to one embodiment of the present invention defines the web of the steel sheet pile as a representative portion. Among the parts of the steel sheet pile, the web has the lowest workability and coarse structure, making it most difficult to secure strength and toughness. Therefore, the microstructure is defined using the web as a representative portion. If the microstructure of the web satisfies the conditions described below, the properties targeted in the present application can be obtained. Therefore, in the steel sheet pile according to one embodiment of the present invention, the microstructure of portions other than the web is not particularly limited. In addition, if the microstructure of the web satisfies the conditions described later, it can be said that there is a high probability that a similar microstructure is obtained in portions other than the web.

As for the microstructure, it is essential that the area ratio, average grain size and maximum grain size of ferrite and the area ratio of island martensite satisfy the following conditions.

[Ferrite main structure]
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. The remaining structure other than ferrite is not particularly limited, but examples include pearlite, bainite structure including island-shaped martensite, and martensite. The total area ratio of the remaining structures other than ferrite is preferably 30% or less. Moreover, the total area ratio of the residual structures other than ferrite is preferably more than 10%. However, the area ratio of island-shaped martensite must be limited as described later. The area ratio of each phase can be measured according to the measuring method described in Examples below.

[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. When the average grain size of ferrite is larger than 15 μm or the maximum grain size is larger than 40 μm, it becomes difficult to ensure toughness. In order to obtain excellent 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. Furthermore, it is more desirable that the ferrite has an average grain size of 10 μm or less and a maximum grain size of 25 μm or less. The lower limits of the average grain size and the maximum grain size of ferrite are not particularly limited. Also, the average grain size and the maximum grain size of ferrite can be measured according to the measurement methods described in Examples below.

[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 better toughness, it is desirable to set the area ratio of island-shaped 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.

Next, a method for manufacturing a steel sheet pile according to one embodiment 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.
Thus, hot rolling includes rough rolling, intermediate rolling and finish rolling. Among these, the rough rolling gives the rough shape of the steel sheet pile. Intermediate rolling refers to rolling from rough rolling to finish rolling (in the above example, claw bending forming (rolling)). Adjust thickness. Finish rolling provides final shape control and includes claw bending in the above example.

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 It is included in a method of manufacturing a steel sheet pile according to one embodiment of the invention. Here, in the straight steel sheet pile 9 shown in FIG.

In hot rolling, the steel material is heated to 1200 ° C. to 1350 ° C., and the average rolling reduction per pass at 900 ° C. to 1150 ° C. is 10% or more, and the cumulative rolling reduction at 800 ° C. to 1150 ° C. is 60% or more, and the finishing temperature of intermediate rolling is 650°C to 900°C. In addition, all the following temperature specifications are based on the surface temperature of the steel material and the material to be rolled. The temperature of the steel material and the material to be rolled can be measured with a radiation thermometer.

[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, solid solution of V in the steel components will be insufficient. As a result, the precipitates become coarse, making it difficult to ensure strength and toughness. In addition, there is a risk that the deformation resistance in the hot state will increase and the rolling rolls will break. On the other hand, if the heating temperature exceeds 1350° C., the crystal grains become coarse and it becomes difficult to ensure toughness. In addition, the heating time increases and the productivity decreases. Therefore, the heating temperature of the steel material is set to 1200°C to 1350°C. The heating temperature of the steel material is preferably 1250°C to 1350°C.

[Average rolling reduction per pass at 900°C to 1150°C is 10% or more]
It is important that the average rolling reduction per pass at 900° C. to 1150° C. (hereinafter simply referred to as the average rolling reduction) is 10% or more. By setting the average rolling reduction to 10% or more, recrystallization of austenite is promoted to obtain uniform and fine crystal grains. This significantly improves YP and toughness. The average rolling reduction is preferably 12% or more.
Note that the average rolling reduction can be calculated by the following formula (1).
R=100{1−( _Tf / _Ts ) ^1/n } (1)
Here, R is the average rolling reduction (%), T _f and T _s are the plate thickness (mm) of the material to be rolled (position corresponding to the web) at 900 ° C. and 1150 ° C., and n is 900 It is the number of rolling passes at °C to 1150°C. As for the number of rolling passes, the number of rolling passes in which at least one of the rolling start temperature and the rolling end temperature is in the range of 900°C to 1150°C is counted. 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 pass in the number of rolling passes. Moreover, the plate thickness can be controlled by the roll gap of the rolling mill.

However, in rolling performed by a plurality of rolling mills such as tandem rolling, it is considered that rolling is continuously performed without time for austenite to recrystallize, and strain is accumulated. Therefore, in such continuous rolling, one pass is considered to be from engagement to disengagement.

Also, the average rolling reduction is preferably 20% or less from the viewpoint of shape control. On the other hand, if the average rolling reduction is less than 10%, austenite recovery or partial recrystallization is remarkable, and crystal grains become coarse and mixed grains, which may make it difficult to ensure toughness.

When hot rolling is performed in a temperature range exceeding 1150° C., grain growth of austenite is large and the effect of refining is small, so the rolling reduction in this temperature range is not particularly specified. Further, if the desired cumulative rolling reduction is obtained in the temperature range of 800° C. to 1150° C., which will be described later, the rolling reduction below 900° C. does not need to be specified either.

[Cumulative rolling reduction at 800°C to 1150°C is 60% or more]
In addition to the above, it is important to set the cumulative rolling reduction at 800° C. to 1150° C. (hereinafter simply referred to as the cumulative rolling reduction) to 60% or more. If the cumulative rolling reduction is less than 60%, the average grain size of ferrite in the microstructure eventually becomes larger than 15 μm, making it difficult to ensure toughness. The cumulative rolling reduction is preferably 70% or more. Although there is no upper limit for the cumulative rolling reduction, the cumulative rolling reduction is preferably 90% or less from the viewpoint of roll breakage risk.
Note that the cumulative rolling reduction can be calculated by the following formula (2).
R'=100 {1-(T _L /T _s )} (2)
Here, R' is the cumulative rolling reduction (%), T _L and T _s are the plate thickness (mm) of the material to be rolled (position corresponding to the web) at 800°C and 1150°C, respectively.

[End temperature of intermediate rolling is 650°C to 900°C]
The end temperature of the intermediate rolling for forming the web or flange (in other words, the end temperature of the final rolling pass of the intermediate rolling) is 650°C to 900°C. When the end temperature of intermediate rolling exceeds 900°C. It becomes difficult to satisfy either of the above two rolling conditions, and eventually the average grain size of ferrite in the microstructure may be larger than 15 μm or the maximum grain size may be larger than 40 μm, making it difficult to ensure toughness. . On the other hand, if the end temperature of the intermediate rolling is less than 650° C., the rolling load in the intermediate rolling increases, increasing the risk of roll breakage in the intermediate rolling mill.
As described above, intermediate rolling refers to rolling from rough rolling to finish rolling (in the above example, nail bending (rolling)). Then, in the intermediate rolling, the thickness is adjusted by rolling down mainly the portion that will become the web in the thickness direction. Also, in finish rolling, final shape control is performed. That is, the finish rolling is not intended only for the final pass of hot rolling, but refers to a rolling process for final adjustment of the shape after intermediate rolling. Finish rolling is performed from the viewpoint of formability, and the conditions for finish rolling are not particularly specified because they do not have a large effect on the properties.

A method for manufacturing a steel sheet pile according to one embodiment of the present invention comprises: during hot rolling (rough rolling, intermediate rolling and finish rolling) and hot rolling (finish rolling (claw bending rolling) for the purpose of improving strength and toughness; )) does not require any subsequent accelerated cooling. That is, 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 hot rolling (finish rolling (claw bending rolling)). From the viewpoint of shape control during rolling, water that is unavoidably applied and cooling such as mist water on the cooling bed do not affect the characteristics of the steel sheet pile.

By adjusting the chemical composition, rolling, and cooling according to the conditions described above, the steel sheet pile can obtain excellent mechanical properties such as high strength of YP 440 MPa or more and vTrs of -10°C or less. In addition, manufacturing conditions other than those described above are not particularly limited, and conventional methods may be followed. For example, the number of rolling passes for rough rolling, intermediate rolling and finish rolling is preferably 5 to 20 passes, 1 to 5 passes and 1 to 3 passes, respectively. Further, rolling passes at 900° C. to 1150° C. include, for example, rolling passes by rough rolling and intermediate rolling. The rolling passes at 800° C. to 1150° C. include, for example, rolling passes by rough rolling and intermediate rolling, and may optionally include rolling passes by finish rolling. The finishing temperature of finish rolling is preferably 550 to 700°C. Further, the steel sheet pile according to one embodiment of the present invention includes a hat shape, a U shape, a combination thereof, a straight shape, etc. regardless of its cross-sectional shape, and the thickness of the web and the shape of the claw portion are not particularly limited. do not have.

Hereinafter, the configuration and effects of the present invention will be described more specifically according to examples. However, the present invention is not limited by the following examples, and can be modified as appropriate within the scope compatible with the spirit of the present invention, all of which are included in the technical scope of the present invention. .

Using a continuous casting machine, a steel material having the steel composition shown in Table 1 (the balance being Fe and unavoidable impurities) was prepared, heated and hot-rolled under the conditions shown in Table 2, and the web shown in FIG. 2, a pair of flanges 3 and 4 extending obliquely from both ends of the web 2, and arms 5 and 6 extending in a laterally widening direction parallel to the web 2, and at both ends of the arms 5 and 6. A hat-shaped steel sheet pile having claws 7 and 8 was manufactured. Cooling after hot rolling was performed by air cooling. In addition, conditions other than the above were assumed to follow the ordinary method.

Observation of the microstructure of the steel sheet pile, tensile test, and toughness test were performed 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 type of structure is identified by cross-sectional observation of the thickness direction of the web at 100 times, and ferrite, pearlite, bainite and martensite are identified by image analysis using a watershed algorithm in a field of view of 800 μm × 600 μm. Each tissue was processed for conversion into three gradations of white, black, and gray 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, and using the equivalent circle diameter of each crystal grain as the grain size of ferrite in the field of view. An average value was obtained. The maximum grain size of ferrite was the maximum value among the circle-equivalent diameters within the field of view. The average grain size of ferrite was calculated using only crystal grains having a circle-equivalent diameter of 3 μm or more that can be confirmed in 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 10 fields of view were randomly selected at a magnification of about 1000 using an SEM. After observing the above, the area ratio of martensite islands was determined by the same image analysis as described above.

<Tensile test>
A JIS No. 1A tensile test piece specified in JIS Z2201 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. (YP) and tensile strength (TS) were determined.

<Toughness test>
A 2 mm V-notch Charpy impact test piece specified in JIS Z2202 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 performed in a temperature range of -80 to 40°C to determine the absorbed energy (vE ₀ ) at 0°C and the fracture surface transition temperature (vTrs) at a ductile fracture ratio of 50%.

Table 2 also shows the results of the above investigation. Test results (Nos. 1 to 17 in Table 2) of steel sheet piles of invention examples manufactured under predetermined manufacturing conditions using compatible steel satisfying a predetermined chemical composition 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. In addition, it was confirmed that none of the invention examples caused a large change in shape such as bending or warping, and could be manufactured stably with high productivity.

On the other hand, comparative examples (Nos. 18 to 38 in Table 2), which did not satisfy the predetermined composition, did not satisfy the predetermined manufacturing conditions, or did not satisfy any of the above, had yield strength and Any value of the fracture surface transition temperature (vTrs) at a ductile fracture surface ratio of 50% does not satisfy the required properties.

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 (4)

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: 0.050-0.300% and N: 0.0010-0.0060%
The balance is Fe and inevitable impurities, and P, S and B as the inevitable impurities are P: 0.025% or less, S: 0.020% or less and B: 0.0003% or less has a component composition of
The microstructure of the web is a ferrite-based structure, the ferrite-based structure has a ferrite area ratio of 70% or more, the ferrite has an average grain size of 15 μm or less and a maximum grain size of 40 μm or less, and The area ratio of island-shaped martensite in the microstructure is 1.0% or less,
A steel sheet pile , wherein the web has a yield strength of 440 MPa or more and a vTrs of -10°C or less. 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,
Nb: 0.005% 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. The microstructure of the web is a ferrite-based structure, the ferrite-based structure has a ferrite area ratio of 70% or more, the ferrite has an average grain size of 15 μm or less and a maximum grain size of 40 μm or less, and A method for manufacturing a steel sheet pile, wherein the area ratio of martensite islands in the microstructure is 1.0% or less, the yield strength in the web is 440 MPa or more, and vTrs is −10° C. or less, ,
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: 0.050-0.300% and N: 0.0010-0.0060%
The balance is Fe and inevitable impurities, and P, S and B as the inevitable impurities are P: 0.025% or less, S: 0.020% or less and B: 0.0003% or less A steel material having a chemical composition is heated to 1200 ° C. to 1350 ° C.,
The steel material is subjected to hot rolling, and in the hot rolling,
Average rolling reduction per pass at 900°C to 1150°C is 10% or more,
The cumulative rolling reduction at 800 ° C to 1150 ° C is 60% or more, and the end temperature of intermediate rolling is 650 ° C to 900 ° C.
A method for manufacturing steel sheet piles. 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,
Nb: 0.005% or less,
4. The method for manufacturing a steel sheet pile according to claim 3, containing one or more of Ti: 0.025% or less and REM: 0.005% or less.

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