JP3314649B2 - High-strength steel with excellent earthquake resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tensile strength 78 excellent in weldability, which is used for various steel structures such as buildings and bridges.
The present invention relates to a 0 MPa class high-strength steel, and particularly to a high-strength steel suitable for use in a structure requiring excellent brittle fracture resistance even under high-speed deformation caused by an earthquake or the like.

[0002]

2. Description of the Related Art Various types of 780 MPa class high-strength steels have been proposed so far, but most of them have been manufactured by adding B for quenching and tempering to improve hardenability. These steel grades have excellent base metal strength and toughness, but since they contain B, the weld is highly susceptible to cracking, so that it is necessary to take measures to prevent weld cracking during welding. Generally, as a measure to prevent welding cracks, the work to be welded is preheated to 100 ° C or higher. However, a work environment heated to a high temperature is not preferable from the viewpoint of safety and health, and the work efficiency is reduced in such an environment. I do. In order to solve such problems of the B-containing 780 MPa class steel, a high-strength steel containing no B is disclosed in Japanese Patent Publication No. 49-42568.

[0003]

However, the high-tensile steel disclosed in Japanese Patent Publication No. 49-42568 has insufficient weldability and weld joint toughness because it contains 0.46% or more of Si. On the other hand, regarding the seismic performance of steel structures, buildings are designed to prevent the collapse of buildings by absorbing the energy of earthquakes by plastic deformation of steel, and the yield ratio is low. Steel materials with excellent plastic deformability are generally used, but bridges and steel towers are designed so that they do not undergo plastic deformation due to seismic motion. Not specifically done.

[0004] However, in the Northridge earthquake of January 1994 in the United States and the Great Hanshin-Awaji earthquake of January 1995,
Since many steel structures including bridges have been severely damaged, various steel structures such as buildings and bridges are required to have even better seismic performance.

[0005] Brittle fracture at a welded joint is one of the characteristic types of fractures observed in the Northridge Earthquake of the United States and the Great Hanshin-Awaji Earthquake. Most steel structures are constructed by welding, but the toe and unwelded portion of the weld metal, welding defects, and the like tend to become stress concentration sources and become fracture starting points. The North Ridge earthquake and the Hanshin-Awaji Earthquake were thought to have been very active and had a very fast shaking speed, and the deformation speed had reached a strain rate of 1 to 10 / s due to the closeness of the epicenter. I have.

[0006] It is said that when a steel material undergoes high-speed deformation, the ductile brittle transition temperature rises as compared with the normal static deformation rate. However, the failures observed in the Northridge earthquake and the Hanshin-Awaji great earthquake are column- It is considered that the high-speed deformation was applied to the stress-concentrated portion such as the beam joint and the scalloped portion, and the ductile-brittle transition temperature in that portion increased, and brittle fracture occurred before the steel material exhibited the plastic deformability. Therefore, based on the conventional concept of seismic performance alone, if an earthquake with a high speed of shaking such as the Northridge earthquake or the Hanshin-Awaji earthquake occurs,
This suggests that building collapse due to brittle fracture cannot be prevented.

[0007] An object of the present invention is to solve the above-mentioned problems by providing a 780 MPa class tensile strength having excellent brittle fracture resistance and excellent weldability even under high-speed deformation caused by an active fault type large earthquake or the like. It is to provide high strength steel.

[0008]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) The steel of the present invention has a C content of 0.07 to 0.1% by weight.
%, Si: 0.01 to 0.4%, and Mn: 0.5 to
1.5%, Ni: 0.5 to 2%, Cr: 0.1 to 1
%, Mo: 0.2-1%, and Nb: 0.003-0.
05%, Al: 0.01 to 0.08%, and N: 0.0
005 to 0.008%, S: 0.005% or less,
O: 0.002% or less and B: 0.0002% or less, satisfy the following formulas (1) and (2), and the balance is F
e and a steel having unavoidable impurities and having a notched value of 30% or more under static loading conditions in a tensile test using a test piece having a notch with a stress concentration coefficient of 5 or more. It is a high-tensile steel with excellent earthquake resistance.

Pcm = C% + Si% / 30 + Mn% / 20 + Cu% / 20 + Ni% / 60 + Cr% / 20 + Mo% / 15 + V% / 10 + 5B% ≦ 0.24% (1) Ceq = C% + Mn% / 6 + Si% / 24 + Ni% / 40 + Cr% / 5 + Mo% / 4 + V% / 14 ≧ 0.45% (2) (2) The steel of the present invention is expressed by weight% and Cu: 0.0%.
1 to 1.5%, V: 0.005 to 0.1%, Ti: 0.
(1) containing at least one member selected from the group consisting of 003 to 0.05%, Zr: 0.005 to 0.1%, and Ca: 0.0005 to 0.005%. It is a high-tensile steel with excellent earthquake resistance as described.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The inventor of the present invention aims at obtaining a high-tensile steel having excellent brittle fracture resistance and excellent weldability even under high-speed deformation caused by an active fault type large earthquake and the like and having excellent tensile strength of 780 MPa. In addition, as a result of intensive studies on the fracture characteristics of steel materials under high-speed deformation, the following findings were obtained.

When a steel material undergoes plastic deformation, the energy required for the plastic deformation is converted to heat energy. However, under high-speed deformation, the time required for heat to be dissipated by heat conduction is short, and the temperature of the steel material rises. Then, the greater the amount of plastic deformation, the greater the amount of heat generated thereby. In general, the higher the temperature, the higher the Charpy absorbed energy of the steel material and the lower the brittle fracture rate.However, if the stress-concentrated parts such as the column-beam joints and scallop are sufficiently plastically deformed even under high-speed deformation, the stress will be reduced. Since the temperature of the concentrated part rises and the ductile-brittle transition temperature rise due to high-speed deformation, that is, the rise of the brittle fracture ratio can be suppressed, it is possible to prevent brittle fracture as seen in the Great Hanshin-Awaji Earthquake etc. .

[0012] However, the plastic deformability of steel materials is based on JIS Z
Generally, it is evaluated based on the drawing value and elongation obtained by a tensile test piece having a parallel portion specified in 2201, but the toe portion of a weld metal found in a column-beam joint, a scalloped portion, and the like. High near the weld, unwelded area, or weld defect 3
Because of the axial stress state, such a stress concentrated portion, that is, the plastic deformation ability in a high triaxial stress state cannot be correctly evaluated by a conventional tensile test piece having a parallel portion. Therefore, as a result of repeated studies on a method of evaluating the plastic deformability under a high triaxial stress state, it was found that a toe portion, an unwelded portion, a weld defect, etc. of a weld metal found at a column-beam joint, a scallop, or the like. It was found that when a tensile test was performed using a notched test piece having a stress concentration coefficient corresponding to the stress concentration state around, the plastic deformation ability at the stress concentration portion could be correctly evaluated by the aperture value at that time.

A notched test piece (FIG. 1,
FIG. 2 shows the relationship between the brittle fracture rate and the temperature when a tensile test was performed using the stress concentration coefficient α = 6.7). Average strain rate between rating points is 0.001 / s (static deformation) and 10 / s
(Corresponding to high-speed deformation at the time of earthquake) under two conditions, but it is clear that the high-speed deformation has a higher brittle fracture ratio and a higher ductile-brittle transition temperature than the static deformation. It can be seen that the fracture behavior under high-speed deformation such as that observed in the Ridge Earthquake and the Hanshin-Awaji Earthquake was reproduced.

Further, if a steel material having a drawing value of not less than a certain value when a tensile test is carried out under static conditions using the above-mentioned notched test piece is sufficiently plastically deformed even under high-speed deformation, The temperature of the stress concentration portion rises, and it becomes possible to suppress brittle fracture.

Here, the resistance of steel to brittle fracture has been evaluated by a Charpy impact test or the like prescribed in JIS Z2242. However, the Charpy impact test cannot compare the difference in fracture behavior depending on the deformation speed, and the resistance to the increase in brittle fracture rate due to high-speed deformation as seen in an earthquake, that is, brittle fracture resistance under high-speed deformation It is impossible to evaluate In order to improve the brittle fracture resistance under high-speed deformation, it is important to increase the plastic deformability under a high triaxial stress state. For example, the fracture surface transition temperature (vT
Even if the rs) is low, that is, even if the steel has high toughness, it is difficult to suppress an increase in the brittle fracture surface rate during high-speed deformation if the drawing value of the notched specimen is low.

It has been known for some time that sulfide and oxide inclusions have an adverse effect on the plastic deformability, ie, ductility, of steel. Has been reduced. However, the periphery of the weld metal toe, unwelded part, weld defect, etc. found at the column-beam joint or scalloped part is in a state of high triaxial stress, so that it can be evaluated by a normal tensile test. Even if the amount of S or O is such that the elongation and the drawing value do not decrease, the sulfide-based and oxide-based inclusions serve as the starting point of microvoids and ductile cracks are easily developed, so that sufficient plastic deformation capacity is obtained. May not be obtained. Therefore, in order to increase the plastic deformability at the stress concentration portion, it is necessary to severely limit the amount of S or O.

Further, in order to realize a high strength with a tensile strength of 780 MPa or more and excellent weldability, steel components are strictly limited, and Pcm = C% + Si% / 30 + Mn% /
20 + Cu% / 20 + Ni% / 60 + Cr% / 20 + M
Pcm defined as o% / 15 + V% / 10 + 5B%
And Ceq = C% + Mn% / 6 + Si% / 24 + N
This can be achieved by limiting the Ceq value defined by i% / 40 + Cr% / 5 + Mo% / 4 + V% / 14 to within a certain value.

Based on the above findings, the present inventor has proposed that S
The amount of oxygen or the amount of O is strictly limited, the Pcm value (weld crack susceptibility index) and the Ceq value (carbon equivalent) are limited to certain values, and a static notch test using a notched test piece with a specified stress concentration coefficient is used. By controlling the drawing value in a tensile test under a loading condition to a certain value or more, the high strength of the present invention having excellent brittle fracture resistance under high-speed deformation, high tensile strength of 780 MPa or more, and excellent weldability. The present inventors have found a tensile steel and completed the present invention. That is, the present invention limits the steel composition, the Pcm value (weld crack susceptibility index), the Ceq value (carbon equivalent), and the tensile properties (drawing value) under the static loading condition by a notched test piece to the following ranges. Excellent brittle fracture resistance even under high-speed deformation caused by active fault type large earthquakes, etc., and excellent tensile strength of 780MP with weldability
A high-strength steel excellent in a-class earthquake resistance can be obtained.

The reasons for adding the components of the present invention, the reasons for limiting the components, and the reasons for limiting the plastic deformation characteristics in the stress concentrated portion will be described below. (1) Component composition range C: 0.07 to 0.1% C is an element necessary for securing the base material strength and the joint strength. %
If added in excess of, the weldability is impaired, so its content is 0.07-0.1%. Si: 0.01 to 0.4% Si is required as a deoxidizing agent in the steel making process while increasing the base metal strength and the joint strength, but if it is less than 0.01%, the effect is insufficient. %, The weldability and the weld toughness are deteriorated, so the content is 0.01 to 0.4%.

Mn: 0.5-1.5% Mn is added to enhance the base metal strength and the joint strength, but if it is less than 0.5%, the strength is insufficient, and if it exceeds 1.5%, it is added. In addition to impairing weldability, the center segregation increases and the toughness at the center of the sheet thickness deteriorates.
~ 1.5%. Ni: 0.5 to 2% Ni is an extremely effective element for improving the base metal strength, joint strength and toughness. However, if it is less than 0.5%, the effect cannot be obtained and it is a very expensive element. From this fact, if it exceeds 2%, it is disadvantageous in terms of cost, so its content is 0.5 to 2%. Cr: 0.1-1% Cr is added to increase the base metal strength and the joint strength. However, if it is less than 0.1%, the strength is insufficient, and if it exceeds 1%, the weldability decreases. Its content is 0.1-1
%. Mo: 0.2 to 1% Mo is an element effective for improving the strength similarly to Cr,
If it is less than 0.2%, the strength is insufficient, and if it exceeds 1%, not only the weldability is lowered but also the cost is disadvantageous, so the content is 0.2 to 1%. Nb: 0.003 to 0.05% Nb is an element that finely precipitates as Nb (C, N) and contributes to an increase in base material strength and joint strength.
%, The effect cannot be obtained, and if added over 0.05%, the weldability or the toughness of the welded portion is reduced.
Its content is 0.003-0.05%. Al: 0.01 to 0.08% Al is necessary as a deoxidizing agent, and is added to secure base metal toughness by refining the microstructure. However, if the content is less than 0.01%, it is insufficient to secure the base material toughness by refining the structure, and if the content exceeds 0.08%, the base material toughness is adversely affected. .0
1 to 0.08%. N: 0.0005% to 0.008% N is an element that reacts with Al to form precipitates, thereby refining the microstructure and improving the base material toughness.
If it is less than 0.0005%, the amount of precipitates is insufficient, and
If the content exceeds 8%, the base material toughness is impaired, so the content is 0.0005 to 0.008%. S: 0.005% or less S is an element that forms sulfide-based inclusions, but in a high triaxial stress state such as a stress-concentrated portion such as a column-beam joint or a scalloped portion, sulfide-based inclusions are present. Becomes the starting point of microvoid generation and promotes the development of ductile cracks, so that the plastic deformability at the stress concentrated portion is significantly reduced. However, 0.005
% Or less, the upper limit of the content is 0.00%.
5%.

O: 0.002% or less O is present as oxide-based inclusions in the steel. However, as with sulfide-based inclusions, O serves as a starting point of microvoid formation and promotes the development of ductile cracks. The plastic deformability at the stress concentration part is significantly reduced. However, since there is no problem if the content is 0.002% or less, the upper limit of the content is 0.002%.

B: 0.0002% or less B is an element that significantly deteriorates the weldability even in a trace amount, and its content must be severely restricted. But,
Since there is no problem if the content is 0.0002% (2 ppm) or less, the upper limit of the content is 0.0002%.

Pcm value (weld crack susceptibility index) ≦ 0.2
4% Pcm value (C% + Si% / 30 +) which is an index of weldability
Mn% / 20 + Cu% / 20 + Ni% / 60 + Cr% /
(20 + Mo% / 15 + V% / 10 + 5B%) is suppressed to 0.24% or less in order to reduce the residual heat temperature during welding.

Ceq value (carbon equivalent) ≧ 0.45% Ceq value (C% + Mn% / 6 + Si) which is an index of hardenability
% / 24 + Ni% / 40 + Cr% / 5 + Mo% / 4 + V
% / 14) is 0.45% or more to secure both the base metal strength and the weld joint strength. In the present invention, in addition to the above alloy elements, in order to increase the strength and toughness of steel, Cu,
One or more of V, Ti, Zr, and Ca may be contained, and the reasons for limiting the components are described below. Cu: 0.01 to 1.5% Cu is an element effective for improving strength and toughness.
If the content is less than 1%, the effect cannot be obtained. If the content exceeds 1.5%, not only the hot workability is lowered, but also surface flaws are easily generated. 5%
It is. V: 0.005 to 0.1% V precipitates as VC and contributes to strength improvement.
If the content is less than 5%, the effect cannot be obtained. If the content exceeds 0.1%, the effect is not only saturated, but also the base material toughness and weldability are impaired. 0.
1%. Ti: 0.003 to 0.05% Ti forms TiN and contributes to an increase in strength.
It is an element that suppresses coarsening of the structure of the weld and is effective in improving HAZ toughness. However, if the content is less than 0.003%, the effect cannot be obtained. If the content exceeds 0.05%, on the contrary, the base material toughness and the weld toughness decrease.
03-0.05%. Zr: 0.005 to 0.1% Zr forms a carbonitride and is an element effective for refining crystal grains. However, if the content is less than 0.005%, the effect cannot be obtained. If the content exceeds 0.1%, the effect is saturated and the cost increases.
Between 0.5 and 0.1%.

Ca: 0.0005 to 0.005% Ca is an element that improves the toughness by controlling the shape of the sulfide-based inclusions, but if less than 0.0005%, the effect cannot be obtained. If the addition exceeds 0.005%, the cleanliness of the steel is adversely affected.
05 to 0.005%. P is allowed to be mixed in a range that does not impair the effects of the present invention.

By adjusting to the above component composition range, a high plastic deformability can be obtained in the stress concentrated portion, so that the heat generation amount at the time of high-speed deformation is large, the brittle fracture resistance is excellent, and the weldability is excellent. It is possible to obtain a high-tensile steel having a tensile strength of 780 MPa. The high-strength steel having such characteristics further has a tensile characteristic under a static loading condition by the following notched test piece.

(2) Tensile Properties of Notched Specimen under Static Loading Conditions In order to obtain the above excellent brittle fracture resistance, a statically loaded specimen having a stress concentration coefficient of 5 or more is used. It is necessary that the aperture value when a tensile test is performed under the conditions be 30% or more.

A test piece having a notch with a stress concentration coefficient of 5 or more is used for a test piece having a notch with a stress concentration coefficient of less than 5 or a test piece without a notch under high-speed deformation (1/3 at a strain rate). This is because it is impossible to evaluate the brittle fracture resistance in (seconds or more). Also, the reason for limiting the drawing value when performing a tensile test under static loading conditions to 30% or more is that if the drawing value in a notch tensile test is less than 30%, the plastic deformation ability of the stress concentrated portion is not sufficient. It cannot be said that under high-speed deformation caused by an earthquake, the temperature rise at the stress-concentrated part seen at the column-beam joint and scalloped part is small, and the brittle transition temperature rises, that is, the brittle fracture surface increases, resulting in brittleness. This is because breakage is likely to occur. In addition, any notch test piece can be used as long as the stress concentration coefficient is 5 or more.

By adjusting the above composition range and tensile properties (aperture value) under static loading conditions using notched test pieces, brittle fracture resistance properties even under high-speed deformation caused by an active fault type large earthquake or the like. It is possible to obtain a high-tensile steel having excellent tensile strength and excellent weldability and a tensile strength of 780 MPa class.

In the present invention, a predetermined strength is obtained by quenching and tempering using steel having the above components. The production conditions at this time are not limited, but in order to obtain a high strength of 780 MPa or more, heating to a temperature of 1000 to 1250 ° C., after rolling, quenching directly from the Ar ₃ transformation point or higher, or quenching after reheating. And then tempering at a temperature equal to or lower than the Ac ₁ transformation point. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0031]

EXAMPLES Steel having the components shown in Table 1 (Steel Nos. 1 to 5 of the present invention)
14, comparative steel No. 15-24) were melted, hot-rolled, and then directly quenched and tempered to produce a steel plate having a thickness of 38 mm. At this time, the heating temperature during hot rolling is 1150 ° C,
The direct quenching temperature was 900-950 ° C and the tempering temperature was 600-650 ° C. About these boards, JI
The yield stress and tensile strength of the material measured by the tensile test of S.Z2201 are shown. In addition, the tensile test piece and the impact test piece at this time were sampled from the direction parallel to the rolling direction at the position of 1/4 of the plate thickness. As the weldability test, JIS Z31
Oblique y crack test specified in 58 and JIS Z31
A maximum hardness test specified in No. 58 was performed. Here, in the oblique y crack test, the presence or absence of crack generation was used as an evaluation criterion.

Table 2 summarizes the results of these tests. The steel of the present invention, No cracks were observed in any of the samples Nos. 1 to 14 in the y crack test, and the value of the maximum hardness (HVmax) of the weld heat affected zone was HV345 or less, indicating that the weldability was excellent. on the other hand,
No. which is a comparative steel. Steels of Nos. 15 to 20 have C, Nb, B content, Pcm, or Ceq out of the range of the present invention.
Also, the value of the maximum hardness (HVmax) of the weld heat affected zone is high. In addition, the comparative steel No. As for 15, 18, and 20, cracks occurred in the y-crack test, so that the weldability was poor. In addition, the comparative steel No. Regarding 21 to 24, the S or O content is out of the range of the present invention, but the other components are in the range of the present invention, so that they show excellent properties in terms of weldability.

Next, the steel No. of the present invention was prepared. Nos. 1 to 14 and Comparative Steel Nos. Test pieces having notches with a stress concentration coefficient of 6.7 as shown in FIG. At this time, the test piece sampling direction was also set at a position of 1/4 of the plate thickness and parallel to the rolling direction. Then, the average strain rate between the scores is 0.1.
001 / sec static tensile test and average strain rate 10 /
A high-speed tensile test was performed for sec to measure the tensile strength, the drawn value, and the brittle fracture surface ratio. Then, the brittle fracture resistance was evaluated from the amount of change in the brittle fracture ratio due to high-speed deformation. The test was performed at -40 ° C.

The results are summarized in Table 3. The steel of the present invention, Nos. 1 to 14 all have a reduction value of 30% or more in a static tensile test, and the brittle fracture rate does not change or decreases in a high-speed tensile test. It is clear that the alloy has excellent brittle fracture resistance. On the other hand, the comparative steel No. 2
All the components 1 to 24 are out of the scope of the present invention,
Since the drawing value in the static tensile test is also smaller than the range of the present invention, the brittle fracture rate is greatly increased in the high-speed tensile test.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

As described above, according to the present invention, the steel composition, the Pcm value (weld crack susceptibility index), the Ceq value (carbon equivalent), and the tensile characteristics (drawing) of the notched test piece under static loading conditions. Value), the phenomenon that the brittle fracture rate of the stress concentrated portion does not increase even under high-speed deformation does not occur, so that the tensile strength is 780 MPa class which is excellent in brittle fracture resistance and excellent in weldability. Therefore, it can be said that the steel material is suitable for use in a steel structure subject to high-speed deformation due to an earthquake or the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing a shape of a tensile test piece according to an example of the present invention.

FIG. 2 is a diagram showing a relationship between temperature and brittle fracture rate in a tensile test according to the embodiment of the present invention.

Continuation of front page (56) References JP-A-3-60894 (JP, A) JP-A-6-116636 (JP, A) JP-A-6-340923 (JP, A) JP-A-10-102197 (JP, A) , A) JP-A-9-324239 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] (1) C: 0.07 to 0.1% by weight
Si: 0.01 to 0.4%, Mn: 0.5 to 1.5%
Ni: 0.5 to 2%; Cr: 0.1 to 1%;
o: 0.2-1% and Nb: 0.003-0.05%
, Al: 0.01 to 0.08%, and N: 0.0005
~ 0.008%, S: 0.005% or less, O: 0.
002% or less and B: 0.0002% or less,
In a tensile test using a test piece that satisfies the following formulas (1) and (2), the balance being Fe and unavoidable impurities and having a notch with a stress concentration coefficient of 5 or more, High-strength steel with excellent earthquake resistance, characterized by having an aperture value of 30% or more under loading conditions. Pcm = C% + Si% / 30 + Mn% / 20 + Cu% / 20 + Ni% / 60 + Cr% / 20 + Mo% / 15 + V% / 10 + 5B% ≦ 0.24% (1) Ceq = C% + Mn% / 6 + Si% / 24 + Ni% / 40 + Cr% / 5 + Mo% / 4 + V% / 14 ≧ 0.45% (2) 2. In% by weight, Cu: 0.01 to
1.5%, V: 0.005 to 0.1%, Ti: 0.00
3 to 0.05%, Zr: 0.005 to 0.1%, and C
a: 1 selected from the group of 0.0005 to 0.005%
The high-strength steel excellent in earthquake resistance according to claim 1, comprising one or more kinds.