JP2014118629A - Steel sheet pile and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet pile having yield stress of 430 MPa or more and to provide its manufacturing method.SOLUTION: There is provided a steel sheet pile containing, by mass%, Nb:0.040 to 0.050%, C, Si and Mn with carbon equivalent Ceof 0.260 to 0.460, Al: limited less than 0.05%, and one or more kind of Cu, Ni, Mo, Cr and V if needed and having a metallographic structure constituted by ferrite-pearlite, the number density of Nb carbonitride of 0.10 to 0.30/μmand yield stress of 430 MPa or more. Ce=[C]+f(C)×{[Mn]/6+[Si]/24+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5}, f(C)=0.75+0.25×tanh{20×([C]-0.12)} There is also provided a manufacturing method of conducting hot rolling on the steel slab of the component with a heating temperature of 1100 to 1300°C, an accumulated rolling reduction rate at 900°C or higher of 90% or more, a finishing temperature of 850°C or higher and cooling.

Description

  TECHNICAL FIELD The present invention relates to a steel sheet pile used for earth retaining and water retaining in the field of civil engineering and construction and a method for producing the same.

  Steel sheet piles with cross-sectional shapes such as hat-shaped, U-shaped, Z-shaped, straight-lined, H-shaped, etc. and having connecting parts (joints) at both ends are widely used as steel materials for earth retaining and water stopping in the field of civil engineering architecture It is used. Since steel sheet piles are required to have weldability and toughness, a technique for limiting the carbon equivalent and adding an alloying element that has a small adverse effect on toughness has been proposed (see, for example, Patent Documents 1 to 3).

  Moreover, when using a steel sheet pile for the revetment in a deep harbor and a soft ground, a steel sheet pile receives high stress. Therefore, in recent years, a steel sheet pile having a yield stress exceeding 430 MPa has been demanded. In the case of manufacturing a steel sheet by reducing the alloy cost and omitting the manufacturing process, if controlled rolling is employed to achieve high strengthening, upper warping or lower warping may occur. In order to solve such a problem, a method of controlling the shape by rolling conditions and cooling conditions has been proposed (for example, see Patent Document 4).

  On the other hand, when hot rolling is performed at a low temperature as in controlled rolling, the load on the rolling roll increases. In particular, when a steel material having a cross-sectional shape such as a steel sheet pile is manufactured by hot rolling, if the deformation resistance of the steel material is high, the load increases, and the rolling roll may break. For such a problem, a steel sheet pile manufacturing method has been proposed in which the Al content is increased, a part of the steel structure is ferrite transformed at high temperature, and the deformation resistance is reduced to perform hot rolling (for example, , Patent Document 5).

Japanese Patent Laid-Open No. 09-287052 Japanese Patent Laid-Open No. 10-001721 JP 2003-253379 A JP 2006-249513 A JP 2007-332414 A

  Since steel sheet piles are required to have weldability and toughness, it is necessary to reduce the carbon equivalent. On the other hand, in order to produce a high-strength steel sheet pile, it is necessary to increase the strength by addition of alloy elements that have a small adverse effect on toughness or controlled rolling. In order to reduce the load on the rolling roll and increase productivity, it is necessary to perform hot rolling by increasing the rolling reduction at high temperatures. However, it is difficult to secure strength by using a hard phase such as bainite. become.

  In view of such circumstances, the present invention solves the conflicting problems of alloy saving, easy manufacturability, and high performance, requires weldability and toughness, and produces a high-strength steel sheet pile in terms of cost and productivity. The steel sheet pile with a yield stress of 430 MPa or more and a method for producing the same are provided.

  The inventors limit the carbon equivalent, add alloying elements, and control precipitates, so that the strength can be reduced without reducing toughness even when hot rolling is performed on steel materials at high temperature and high pressure. The method of improvement was examined. As a result, when performing hot rolling at a high rolling reduction rate, the amount of Nb is optimized to control the precipitation of Nb carbonitride, and the precipitation strengthening by precipitation particles is used to strengthen the ferrite-pearlite structure. Thus, the present invention was completed by finding that a steel sheet pile having a yield strength of 430 MPa or more can be produced without significantly impairing toughness.

The gist of the present invention is as follows.
[1] By mass%
C: 0.05 to 0.18%,
Si: 0.10 to 0.50%,
Mn: 0.50 to 1.50%
Nb: 0.040 to 0.050%
Containing
Al: limited to less than 0.05%, the balance is made of Fe and inevitable impurities, the metal structure is made of ferrite-pearlite, and the number density of Nb carbonitride is 0.10-0.30 pieces / μm ² , the following (equation 1) and a carbon equivalent Ce _N obtained by equation (2) is 0.260 to 0.460, steel sheet pile, characterized in that the yield strength is not less than 430 MPa.
Ce _N = [C] + f (C) × {[Mn] / 6 + [Si] / 24 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5} (Formula 1)
f (C) = 0.75 + 0.25 × tanh {20 × ([C] −0.12)}
... (Formula 2)
Here, [C], [Mn], [Si], [Ni], [Cr], [Mo], [Nb], and [V] are the contents (mass%) of each element and are not contained. The element is 0.
[2] By mass%,
Cu: 0.05 to 0.40%,
Ni: 0.10 to 1.00%,
Mo: 0.10 to 1.00%,
Cr: 0.10 to 1.00%,
V: 0.05-0.20%
The steel sheet pile according to the above [1], comprising one or more of the above.
[3] The steel sheet pile according to [1] or [2], wherein a crystal grain size of the metal structure is 10 to 80 μm.
[4] A steel slab comprising the component described in [1] or [2] above is heated to 1100 to 1300 ° C, the cumulative rolling reduction of 900 ° C or higher is 90% or higher, and the finishing temperature is 850 ° C or higher A method for producing a steel sheet pile, characterized by performing hot rolling and cooling.

  According to the present invention, the yield stress is reduced without excessive addition of the alloy, and after improving the productivity and preventing breakage of the rolling roll by hot rolling with an increased reduction rate at a high temperature. It is possible to provide a high-strength steel sheet pile of 430 MPa or more and a manufacturing method thereof, and the industrial contribution is extremely remarkable.

It is a figure explaining the relationship between a crystal grain diameter, intensity | strength, and elongation.

  In order to secure the strength and toughness of the steel sheet pile base metal and the toughness of the welded portion, it is extremely important to control the hardenability, that is, to optimize the content of C and other alloy components that enhance the hardenability. . The hardenability is evaluated by the carbon equivalent, and a relational expression for obtaining the carbon equivalent from the alloy component has been proposed. In the case of steel sheet piles, in particular, since hardenability is limited in order to ensure the toughness of the welded portion, it is necessary to lower the hot rolling temperature to improve the strength of the base material.

  On the other hand, it is desirable to perform hot rolling at a high temperature in consideration of productivity improvement and load on the rolling roll. Therefore, it is difficult to achieve both high strength and high toughness of the steel sheet pile base material and high toughness of the welded portion only by adjusting the hardenability. Therefore, the present inventors examined securing of strength by precipitation strengthening. In general, precipitation strengthening impairs toughness, but as a result of the study by the present inventors, by controlling the amount of Nb added, even if hot rolling is performed with a reduction rate of 900 ° C. or higher, Nb carbonitride It has been found that the precipitation of is promoted and the coarsening of the particle size is suppressed.

  The inventors have further studied, optimized the amount of Nb added and hardenability, and controlled the precipitation of Nb carbonitride to improve the temperature and hot rolling temperature from the viewpoint of productivity and rolling load. Even if the reduction ratio was increased, a high strength steel sheet pile was successfully obtained without impairing the toughness of the base metal and the weld.

In the present invention, since the addition of Nb, an indication of the hardenability following (Equation 1) and the carbon equivalent Ce _N obtained by Equation (2). Wherein the carbon equivalent Ce _N is a known equation.
Ce _N = [C] + f (C) × {[Mn] / 6 + [Si] / 24 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5} (Formula 1)
f (C) = 0.75 + 0.25 × tanh {20 × ([C] −0.12)}
... (Formula 2)
Here, [C], [Mn], [Si], [Ni], [Cr], [Mo], [Nb], and [V] are the contents (mass%) of each element and are not contained. The element is 0.

  Hereinafter, the present invention will be described in detail. First, the components of the steel sheet pile of the present invention will be described. Here, “%” for a component means mass%.

  C is an element effective for increasing the strength of steel. In the present invention, in order to ensure the strength, the lower limit of the C content is set to 0.05% or more. On the other hand, when C is contained excessively, the toughness is lowered. Therefore, in the present invention, the upper limit of the C content is 0.18% or less. In order to improve the balance between strength and toughness, the lower limit of the C content is preferably 0.10% or more.

  Si is a deoxidizing element, and the lower limit of the content is 0.10% or more. Si is also an element that improves strength, and preferably contains 0.20% or more. On the other hand, since the toughness deteriorates when the Si amount becomes excessive, the upper limit of the Si amount is set to 0.50% or less.

  Mn is an element that improves the hardenability of steel and is useful for ensuring strength and toughness. In the present invention, in order to ensure strength, the lower limit of the amount of Mn is 0.50% or more. On the other hand, when the amount of Mn is excessive, the hardenability is increased and the toughness is lowered. Therefore, in the present invention, the upper limit of the amount of Mn is set to 1.50% or less. In order to improve the balance between strength and toughness, the lower limit of the amount of Mn is preferably 0.80% or more.

  Nb is an extremely important element in the present invention, and 0.040% or more is added to ensure strength by precipitation strengthening of Nb carbonitride. On the other hand, when Nb exceeding 0.050% is added, the toughness of the base material is lowered by Nb carbonitride, and the toughness of the welded portion is impaired due to the increase in hardenability, so the upper limit of Nb amount is 0.050% or less. To do.

  Al is a deoxidizing element, but when Si is used for deoxidation, since it is not always necessary to add Al, the lower limit of the amount of Al is not particularly limited. On the other hand, when the amount of Al is excessive, coarse oxides are formed and the toughness is lowered. Therefore, the upper limit of the amount of Al is limited to less than 0.05%. The upper limit of the amount of Al is preferably 0.03% or less, and more preferably 0.02% or less.

Without lowering the toughness of the weld, in order to ensure the strength of the base material, the carbon equivalent Ce _N calculated by the following (formula 1) is 0.260 to 0.460. Carbon equivalent Ce _N is an index of hardenability, in order to ensure a more yield stress 430 MPa, shall the lower limit above 0.260. Meanwhile, in order to ensure the toughness of the base material and welds, it is necessary that the carbon equivalent Ce _N to 0.460.
Ce _N = [C] + f (C) × {[Mn] / 6 + [Si] / 24 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5} (Formula 1)
f (C) = 0.75 + 0.25 × tanh {20 × ([C] −0.12)}
... (Formula 2)
Here, [C], [Mn], [Si], [Ni], [Cr], [Mo], [Nb], and [V] are the contents (mass%) of each element and are not contained. The element is 0.

  Furthermore, you may add 1 type (s) or 2 or more types of Cu, Ni, Mo, Cr, and V as a selection component in a base material as needed.

  Cu is an element that improves the strength by solid solution in steel, and it is preferable to add 0.05% or more. On the other hand, since excessive addition of Cu may cause precipitation of CuS and deterioration of surface properties, the upper limit of Cu content is preferably set to 0.40% or less. More preferably, the upper limit of Cu content is 0.30% or less.

  Ni is an element that improves hardenability and improves the strength and toughness by solid solution in steel, and it is preferable to add 0.10% or more. On the other hand, since Ni is an expensive element, the upper limit of the content is preferably set to 1.00% or less. More preferably, the upper limit of the Ni amount is 0.50% or less, and more preferably, the upper limit of the Ni amount is 0.30% or less. In addition, when adding Cu, in order to prevent deterioration of a surface property, it is preferable to add Ni simultaneously.

  Mo is an element that improves the strength by dissolving in steel, and it is preferable to add 0.10% or more. On the other hand, Mo is an element that increases the high-temperature strength, and when added excessively, deformation resistance increases and there is a concern about roll breakage, so the upper limit of the Mo amount is preferably 1.00% or less. More preferably, the upper limit of the Mo amount is 0.50% or less, and still more preferably, the upper limit of the Mo amount is 0.30% or less.

  Cr is an element that increases the hardenability of the steel and is effective in increasing the strength, and it is preferable to add 0.10% or more. On the other hand, if Cr is added excessively, the toughness of the welded part and the base metal may be deteriorated, so the upper limit of Cr content is preferably made 1.00% or less. More preferably, the upper limit of the Cr amount is 0.50% or less, and still more preferably, the upper limit of the Cr amount is 0.30% or less.

  V is an element that forms a compound with C and N and improves the strength of the steel material, and it is preferable to add 0.05% or more. On the other hand, since excessive addition of V may adversely affect the toughness of the base material, the upper limit of the V content is preferably 0.20% or less.

  Next, the metal structure of the steel sheet pile of the present invention will be described.

  The metal structure of the steel sheet pile of the present invention is composed of ferrite-pearlite and further contains precipitates. Precipitates are Nb carbonitrides such as Nb (C, N), and ensure toughness and improve strength by precipitation strengthening by fine precipitates and suppression of coarsening of the structure by pinning. .

If the number of Nb carbonitrides per unit area is less than 0.10 pieces / μm ² , sufficient strength cannot be obtained, and if it exceeds 0.30 pieces / μm ² , the toughness is lowered. Therefore, the number density of Nb carbonitride is set to 0.10 to 0.30 / μm ² . Nb carbonitride can be measured with a transmission electron microscope using an extracted replica as a sample.

  The properties of the steel sheet pile are affected by the crystal grain size of the metal structure, and when the crystal grain size exceeds 80 μm, the toughness and strength tend to decrease. On the other hand, when the crystal grain size is less than 10 μm, the elongation tends to decrease. Accordingly, the crystal grain size is particularly preferably 10 to 80 μm for obtaining good characteristics, but the present invention is not limited to this crystal grain size range. The metal structure of the steel sheet pile of the present invention can be observed with an optical microscope, and the crystal grain size can be determined according to JIS G 0551.

  FIG. 1 shows an example of the relationship between crystal grain size, strength, and elongation based on test results using some samples. When the crystal grain size is larger than 80 μm, for example, when it exceeds 100 μm, the yield strength may be less than 430 MPa, and when the crystal grain size is less than 10 μm, the elongation may be reduced.

  Next, the manufacturing method in the steel sheet pile of this invention is demonstrated.

  In the steel making process, the chemical composition of the molten steel is adjusted by a conventional method and then cast to obtain a steel piece. From the viewpoint of productivity, the casting is preferably continuous casting. The thickness of the steel slab is preferably 200 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of reduction of segregation and time required for heating.

  The steel sheet pile of the present invention is manufactured by hot rolling a steel piece. After hot rolling, air cooling may be performed, but accelerated cooling may be performed in order to increase strength and toughness.

  If the heating temperature of the steel slab is too low, the temperature of the steel material is lowered during hot rolling, and the deformation resistance becomes too high. On the other hand, if the heating temperature of the steel slab exceeds 1300 ° C, the load on the heating device increases and the scale generated on the surface of the steel slab increases, so the upper limit is made 1300 ° C or lower.

  After the steel slab is heated, hot rolling is performed. In hot rolling, in order to improve productivity, reduce the load on the roll and prevent breakage, the cumulative rolling reduction in the range of 900 ° C. or higher is set to 90% or higher. In the present invention, the amount of Nb is optimized so that fine Nb carbonitride is precipitated. Therefore, even if hot rolling with such a high rolling reduction is performed, the effect of precipitation strengthening and pinning is achieved. Thus, toughness and strength can be ensured. When the cumulative strain reduction ratio on the lower temperature side is increased in the range of 900 ° C. or higher, the structure of ferrite and pearlite is refined, and the strength and toughness can be increased. When the finishing temperature exceeds 900 ° C., the cumulative rolling reduction in the range above the finishing temperature is 90% or more.

  The finishing temperature of hot rolling is 850 ° C. or higher. This is because, when hot rolling is performed at a temperature lower than 850 ° C., ferrite transformation has started, and therefore, two-phase rolling is performed. When two-phase rolling is performed, processed ferrite is generated, the toughness of the base material is deteriorated, the deformation resistance is increased, and the load on the roll is increased. When the finishing temperature is 920 ° C. or higher, the particle size may increase and elongation may decrease. The finishing temperature is preferably less than 920 ° C, more preferably 910 ° C or less, and still more preferably 900 ° C or less.

  Steel pieces having the components shown in Table 1 were produced by continuous casting. The obtained steel slab was heated in a heating furnace and subjected to hot rolling. The manufacturing conditions are shown in Table 2. A test piece is taken from a position (1/6 W) of the web width of the obtained steel sheet pile, a tensile test is performed according to JIS Z 2241, and a Charpy impact test is performed according to JIS Z 2242. It was. The mechanical properties were set such that the yield strength YP and the tensile strength TS were 430 MPa or more and 510 MPa or more, respectively, and the impact value was 43 J or more.

  Further, a sample was taken from a 1/6 W portion, and the structure was observed with an optical microscope to confirm that the metal structure was ferrite-pearlite, and the particle size was measured. Furthermore, the extracted replica sample was observed by TEM to determine the number density of Nb carbonitrides in the structure. The measurement was performed in an area of 10 μm × 10 μm. The results are shown in Table 2.

  No. Examples 1 to 8 are examples, and all satisfy the materials. In addition, No. No. 8 has a higher finishing temperature, a slightly larger particle size, and a lower elongation.

On the other hand, no. 9 to 20 are comparative examples, and the strength and Charpy absorbed energy have not reached the target values. In Nos. 9, 11, 13, 15, and 17, any of C, Mn, Si, Nb, and Al is large, and the impact value is low. On the other hand, no. In 10, 12, 14, and 16, any of C, Mn, Si, and Nb is small, and the yield strength is lowered. No. with a large amount of Nb. No. 15 has a density of Nb carbonitride that is excessive and Nb content is small. No. 16 lacks the density of Nb carbonitride. No. In No. 19, Ce _N was too high and the toughness was lowered. 18 and no. In No. 20, Ce _N is too low, and the yield strength is lowered.

Claims (4)

% By mass
C: 0.05 to 0.18%,
Si: 0.10 to 0.50%,
Mn: 0.50 to 1.50%
Nb: 0.040 to 0.050%
Containing
Al: limited to less than 0.05%, the balance is made of Fe and inevitable impurities, the metal structure is made of ferrite-pearlite, and the number density of Nb carbonitride is 0.10-0.30 pieces / μm ² , the following (equation 1) and a carbon equivalent Ce _N obtained by equation (2) is 0.260 to 0.460, steel sheet pile, characterized in that the yield strength is not less than 430 MPa.
Ce _N = [C] + f (C) × {[Mn] / 6 + [Si] / 24 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5} (Formula 1)
f (C) = 0.75 + 0.25 × tanh {20 × ([C] −0.12)}
... (Formula 2)
Here, [C], [Mn], [Si], [Ni], [Cr], [Mo], [Nb], and [V] are the contents (mass%) of each element and are not contained. The element is 0. In mass%,
Cu: 0.05 to 0.40%,
Ni: 0.10 to 1.00%,
Mo: 0.10 to 1.00%,
Cr: 0.10 to 1.00%,
V: 0.05-0.20%
The steel sheet pile according to claim 1, comprising one or more of the following.   The steel sheet pile according to claim 1 or 2, wherein a crystal grain size of the metal structure is 10 to 80 µm.   The steel slab comprising the component according to claim 1 or 2 is heated to 1100 to 1300 ° C, subjected to hot rolling at a cumulative reduction rate of 900 ° C or higher of 90% or higher and a finishing temperature of 850 ° C or higher, and cooled. The manufacturing method of the steel sheet pile characterized by doing.

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