JP3434444B2 - Steel plate for hull with excellent shock absorption capacity - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate for a hull having a plate thickness of 8 mm or more, and particularly, in the event of a collision accident of a ship represented by an oil spill accident due to a collision of a tanker, the destruction of the accident will occur. The present invention relates to a steel plate for a hull excellent in shock absorption capacity that can be stopped to a minimum.

[0002]

2. Description of the Related Art In recent years, oil has been spilled out due to a marine accident such as a collision of a tanker or a grounding, causing marine pollution and becoming a social problem.

In the marine accidents of tankers, the oil spill rate of tankers of 60,000 tons or more is particularly high.
There are few oil spill accidents due to marine accidents of small tankers of 60,000 tons or less.

In a small tanker collision accident, it is unlikely that a large-scale oil spill accident will occur because it is unlikely that the tanker that collided will move and weaken the impact to open a hole in the hull due to the small size of the tanker. Conceivable. On the other hand, 6
In the case of a collision accident of a tanker of 10,000 tons or more, a hole opens in the hull of the tanker, causing a large-scale oil spill accident, which causes marine pollution (environmental pollution), which is a major social problem.

Therefore, in the recent field of shipbuilding, in order to prevent marine pollution due to oil spill accidents of tankers,
In the unlikely event of a collision between ships, the technology is being studied to minimize damage to the ship and minimize damage such as oil spills from tankers and flooding from damaged parts.

As one of the techniques, it is considered that the hull steel plate itself has an energy absorbing ability to prevent the hull from being destroyed at the time of a collision of a ship. A steel sheet with improved energy absorption capacity has not been proposed yet.

Other than ships that require safety at the time of collision, collision safety of automobiles is well known, and energy absorption mechanism at the time of collision of automobiles has been studied to absorb energy at the time of collision. Various proposals have been made for automobile structures and automobile steel plates. However, the present situation is that the steel material has not been studied so far regarding the energy absorption capacity at the time of collision of a ship such as a tanker.

That is, the collision energy absorption mechanism of an automobile absorbs energy by a buckling mode under a compressive load. Specifically, it is not a single buckling of a thin steel plate, but various types as a whole. The buckling mode is designed to protect the passengers by gently buckling with the sections of.

On the other hand, the collision energy absorption mechanism of a tanker is shown in FIG. 1 (a)-(c), where one example of collision between tankers is shown in FIG.
When the toe 2 of another tank collides with the side wall 1 of the tanker, first, as shown in (b), the entire toe 2 of the tanker fits into the flat steel plate 1 of the side wall of the tanker. Undergoes large bending deformation, and as shown in (c), the steel sheet 1 is stretched deeply and greatly stretched. If the steel plate is broken (broken) during this collision process, an oil spill in the tanker or seawater inundation accident will occur.

Therefore, in order to prevent oil outflow or the like when the tankers collide with each other, when the steel sheet is largely bent at the initial stage of collision, the steel sheet can withstand the bending, and then the unbent portion is greatly stretched. Although it causes tensile deformation, it is necessary that the part should stretch uniformly and not break.

In this way, in the case of a tanker collision, deformation may occur, but if it does not break and break, an oil spill accident or a flood accident will not occur.

[0012] As described above, automobiles and ships have completely different collision energy absorption mechanisms, and in the case of ships, it is essential to weld steel plates, which seriously damages the hull. The strength of the welded joint, which has a negative effect, must be ensured.

For example, the strength of the thick steel plate for ships is 350 to 550 MPa from the viewpoint of weldability, but the strength of the thin steel plate of the collision safety member for automobiles is 600 to 1000 MPa.
However, because of its high strength and poor weldability, it cannot be used in shipbuilding where arc welding is frequently used due to both the softening problem of the heat affected zone of the weld zone and the weld cracking problem.

In the case of steel plates applied to members of automobiles that are required to have collision safety, the plate thickness used for shipbuilding is 8
Conventionally, the size of mm or more does not exist.

For these reasons, steel sheets for automobiles having completely different properties required for steel sheets cannot be used as they are as steel sheets for hulls.

[0016]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel plate for a hull which is capable of minimizing damage to the hull when a tanker collides with it and has an excellent shock absorption resistance.

[0017]

DISCLOSURE OF THE INVENTION The inventor of the present invention has found that a steel plate for a hull excellent in impact absorption resistance for absorbing a large amount of energy at the time of collision and preventing the hull from breaking is provided with the following properties. Found that it was necessary to stay. (B) The bending part should be sufficiently work-hardened and the subsequent deformation should be shared by the parallel part. (B) In particular, in the welded joint portion, the bending portion, which is likely to occur mainly, is not embrittled by being strained to cause brittle fracture. (C) When the parallel part is stretched, the ductile crack initiation resistance is large. (D) The ductile crack has a large propagation resistance when the parallel portion is stretched. (E) When the parallel part is stretched, it has high strength and elongation. (F) Sufficient work hardening when the parallel part is stretched.

According to the present invention, in order to obtain a steel sheet having improved characteristics of (e) among the above-mentioned properties (a) to (f), which has a great influence on the energy absorption capacity at the time of collision, the composition of the steel and The present invention was completed by researching the structure of steel and the like.

The gist of the present invention is as follows.

(1) In% by weight, C: 0.05 to 0.2%, Si: 0.05 to 1.0%, Mn: 0.5-2.0%, Al: 0.001-0.1%, And carbon equivalentCeq (= C + Mn / 6)
(%) Is 0.42 (%) or less, and the balance Fe and
A steel plate with a thickness of 8 mm or more made of inevitable impurities, and
It has a ferrite-based structure with a space factor of 80% or more.
And the hardness of the ferrite phase is Hv 160 or more.
Steel plate for hulls with excellent impact resistance.

(2) Further, in weight%, S: ≦ 0.0
05% is contained, The steel plate for ship bodies excellent in impact absorption resistance as described in said (1).

(3) Further, in% by weight, Nb: 0.0
The above (1), which contains 01 to 0.1%
And a steel plate for a hull excellent in impact absorption resistance according to any one of (2) and (2).

(4) Further, in weight%, V: 0.001 to 0.1%, Ti: 0.001 to 0.05%, Ta: 0.001 to 0.1%, Cr: 0.01 ~1.0%, Ni: 0.01~1.0%, Mo: 0.01~1.0%, Cu: containing one or more of 0.01% to 1.0% , And carbon equivalent Ce
q (= C + Mn / 6 + (Cu + Ni) / 15 + (V + M
o + Cr) / 5) (%) is 0.42 (%) or less, excellent in impact absorption capacity according to any one of claims 1 to 3. Steel plate for hull.

(5) Further, in% by weight, Ca: 0.0
001-0.01%, Mg: 0.0001-0.01
%, REM: 0.001 to 0.05%, and one or more of them are contained, and the shock absorption according to any one of (1) to (4) above. Steel plate for hulls with excellent performance.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.

The present inventor investigated the relationship between the collision energy absorption capacity of a ship and the mechanical properties of steel.

As a result, collision energy absorption capacity (EA)
And the mechanical properties of steel, the elongation properties (E
It has been found that both L) and strength properties (YP, TS) are excellent in order to improve the collision energy absorption capacity (EA). The mechanical properties were evaluated using the U1 test piece in the Japan Maritime Association Steel Ship Regulations.

Generally, the energy absorption until the material breaks is accurately determined by ∫σdε, where σ is the stress and ε is the displacement, but this is applied to the steel plate and the flow stress as the stress: (YP + TS) / 2 If the elongation at break: EL is used as the displacement, the energy absorption can be approximately expressed as (YP + TS) × EL / 2. The fact that the ship-side deformation behavior at the time of a ship collision is governed by the tensile properties of the members is described in T.W. Ishikawa et al. 1
5th International Confere
nce on OMAE, 1996, Book No.
This is as shown in the numerical simulation of fracture in G00987-1996. Therefore, it can be said that the above-mentioned index has a corresponding relationship with the energy absorption capacity of the hull steel sheet during a hull collision. Therefore, the collision energy absorption capacity EA can be expressed as EA = (YP + TS) / 2 × EL.

When the collision energy absorption capacity (EA) of the currently commonly used hull steel plates was investigated according to the above formula, the EA was about 100 MPa. The goal of the present invention is to obtain the collision energy absorption capacity (E) of this conventional steel sheet.
Larger than A), that is, collision energy absorption capacity 120
It is to obtain a steel sheet having a pressure of at least MPa, especially at least 130 MPa.

Therefore, first, the influence of the elongation property (EL) of the steel material and the structure was investigated. The result is shown in FIG.

The horizontal axis of FIG. 2 is the ferrite space factor, and the vertical axis is the total elongation. In FIG. 2, there are four types of ferrite hardness, that is, F.I. Hv: 160-165, F.I. Hv: 165
~ 170, F.I. Hv: 170-180 and F.I. Hv: 1
It shows the total elongation for steels of 80 and above. As shown in Figure 2, as the ferrite space factor increases, EL
When the characteristics are improved and the ferrite space factor is 80% or more, the elongation characteristics are rapidly improved. Therefore,
In the present invention, it has been found that it is important to have a ferrite structure having a ferrite space factor of 80% or more in order to secure the elongation property (EL).

On the other hand, the strength characteristics of YP and TS are contrary to the elongation characteristics EL, and it is generally difficult to improve both at the same time.

Therefore, the present invention pays attention to the hardness (Hv) of the ferrite phase as a means for increasing the yield point YP + tensile strength TS which is the strength characteristic while ensuring the elongation characteristic EL, and the ferrite hardness (Hv) Hv) strength characteristics (YP + T
I investigated how S) changes. The result is shown in FIG. As shown in FIG. 3, it was found that the strength (YP and TS) was improved as the ferrite hardness (Hv) was increased. Further, as shown in FIG. 4, the relationship between the ferrite hardness (Hv) and the elongation property (EL) was investigated.
The change in ferrite hardness (Hv) had almost no effect on the decrease in elongation property (EL).

As a method of hardening the ferrite phase, S
This can be achieved by setting the content of i to be 0.1% or more or by including a strengthening component such as Nb. Further, as another method, the hardness of the ferrite phase can be increased by rapidly cooling the structure during the ferrite transformation in the rolling process of the steel sheet or after the transformation at a cooling rate of 5 ° C./s or more.

When a strengthening element is added, Nb is preferable, and in order to increase the hardness of the ferrite phase, it can be achieved by, for example, fine precipitation of Nb and / or solid solution of Nb in ferrite. . Specifically, 1
Fine Nb precipitates such as Nb-C can be obtained by sufficiently dissolving Nb in steel at 100 ° C or higher and rolling while securing a reduction rate in the precipitation temperature range of the dissolved Nb (800 ° C or higher). (Solid solution Nb) can be formed in the rolling process of a steel sheet. Here, the term “fine precipitate particles” refers to a state in which precipitates having a particle size of less than 0.1 μm, which cannot be observed with an ordinary electron microscope, are dispersed in ferrite. It can be confirmed by the analysis method.

FIG. 5 shows the relationship between the amount of solid solution Nb (wt%) and the hardness of ferrite (Hv). As shown in FIG. 5, the ferrite hardness increases as the amount of solute Nb increases,
It can be seen that the hardness of the ferrite phase can be secured.

As described above, by increasing the ferrite space factor to improve the elongation characteristic EL and by increasing the hardness of the ferrite phase, the strength characteristic (YP + TS) can be improved while securing the elongation characteristic EL. By increasing, the impact energy EA can be improved as compared with the conventional hull steel plate.

FIG. 6 is a diagram showing changes in impact absorption energy (MPa) with respect to changes in ferrite space factor and ferrite hardness. Ferrite hardness H
In the ferritic steel of v160 or less (comparative steel), when the ferrite space factor is 80% or more, the impact absorption energy increases, but it is about 100 MPa at the maximum.
On the other hand, the steel of the present invention having a ferrite hardness Hv of 160 or more exhibited a shock absorption energy value of 130 MPa or more at a ferrite space factor of 80% or more.

As described above, the ferrite space factor is 80
%, The collision energy absorption capacity (EA) can be secured to 120 MPa or more, particularly 130 MPa or more, which is the target of the present invention, by setting the hardness of the ferrite phase to 160 Hv or more.

Next, the reasons for limiting the components of the steel of the present invention will be explained.

C is the cheapest and most effective element for increasing the strength of the steel sheet, and is 0.05 for securing the strength.
% Or more is required, but if it exceeds 0.2%, weldability and weld joint toughness are deteriorated, which is not preferable. For this reason,
C was 0.05 to 0.2%.

Si is required to be 0.05% or more as a deoxidizing element of steel, but if it exceeds 1.0%, the weldability and weld joint toughness deteriorate, so 0.05 to 1.0% was made. .
Further, in order to increase the hardness of the ferrite phase without quenching in the rolling process, it is preferable to add 0.1% or more.

Mn is required to be 0.5% or more in order to secure the toughness of steel and to increase the strength, but if it exceeds 2.0%, the weldability deteriorates, so 0.5 to 2. It was set to 0%.

Al is the most important element as a deoxidizing element of steel and is required to be 0.001% or more, but if it exceeds 0.1%, the weldability is deteriorated, so 0.001 to 0.1% is set. did.

The steel of the first invention of the present invention has the above-mentioned basic components, but P, which is an impurity element, need not be restricted in particular, but is 0.04% or less from the viewpoint of ensuring toughness and weldability. It is preferable. Further, S, which is an impurity element, does not need to be particularly regulated, but from the viewpoint of securing toughness and elongation, S.
It is preferably set to 04% or less. However, in order to secure the improvement in elongation of the base material, it is important to set S, which is an unavoidable impurity element, to 0.005% or less as in the second invention.

The present invention provides a third invention and a fourth invention in which a selective element which is a reinforcing component of the base material is further added to the above basic component.

The reason for limiting the selected element that is the reinforcing component will be described.

Addition of 0.001% or more of Nb is effective in hardening the ferrite phase and is also effective in increasing the strength of the base material, and has the effect of increasing the ferrite fraction while maintaining the strength. If it exceeds 0.1%, the weldability is deteriorated, so the content was made 0.001 to 0.1%.

V is effective for increasing the strength of the base metal when added in an amount of 0.001% or more, but if it exceeds 0.1%, the weldability is deteriorated, so V was made 0.001 to 0.1%.

When Ti is added in an amount of 0.001% or more, it forms a complex oxide, a Ti nitride, etc. and is effective in increasing the strength of the base material. However, if it exceeds 0.05%, the HAZ toughness deteriorates. Since the weldability is deteriorated, it is set to 0.001 to 0.05%.

Addition of 0.001% or more of Ta is effective for increasing the strength of the base material, but if it exceeds 0.1%, the weldability is deteriorated, so 0.001 to 0.1% was made.

If Cr is added in an amount of 0.01% or more, it improves the hardenability and is effective in securing the strength of the base material. However, if it exceeds 1.0%, it deteriorates the weldability and the low temperature toughness. , 0.1 to 1.0%.

Ni is effective for improving the toughness and strength of the base material when added in an amount of 0.01% or more, but if it exceeds 1.0%, the weldability is deteriorated and the cost increases, so 0.01 to 0.01%
It was 1.0%.

When Mo is added in an amount of 0.01% or more, it is effective in improving the hardenability of the base material and ensuring the strength, but if it exceeds 1.0%, the weldability is deteriorated and the low temperature toughness is deteriorated. , 0.01 to 1.0%.

Cu is effective in increasing the strength of the base metal when added in an amount of 0.01% or more, but if it exceeds 1.0%, hot cracking tends to occur and the weldability is also deteriorated. ~ 1.
It was set to 0%.

Further, in the present invention, the fifth invention is to add a selective element for improving the elongation of the base material.

The reasons for limiting the selection elements for improving elongation will be described.

When Ca is added in an amount of 0.0001% or more, it fixes S, which is harmful to the elongation improvement, and is effective in improving the elongation.
If it exceeds 0.01%, the weldability will deteriorate, so 0.00
It was set to 01 to 0.01%.

Addition of 0.0001% or more of Mg fixes S, which is harmful to the elongation improvement, and is effective in improving the elongation.
If it exceeds 0.01%, the weldability will deteriorate, so 0.00
It was set to 01 to 0.01%.

REM (rare earth element) is effective in fixing S, which is harmful to elongation improvement, by adding 0.001% or more, and is effective in improving elongation, but if it exceeds 0.05%, weldability deteriorates and it is expensive. Therefore, it was set to 0.001 to 0.05%.

Further, in the steel of the present invention, the higher the carbon equivalent (Ceq) according to the following formula is an index showing the effect of the constituent elements on the hardenability of the weld, the easier it is to harden the weld and the cold cracking susceptibility during welding. Can be kept low. Therefore, the upper limit is set to 0.42. If it exceeds this, cracks are likely to occur in the welded portion, and preheating and postheating are required to prevent the cracking, resulting in a significant increase in cost.

Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (V + Mo + Cr) / 5 In the above formula, components not contained in the steel
The element is 0. Further, in the present invention, the thickness of the steel plate is limited to 8 mm or more. This is because the tanker size is 60,000 tons or more which causes a hole in the hull to cause an oil spill accident or a flood accident due to a tanker collision accident. As a tanker of this class of tanker, it is necessary to use a steel plate having a plate thickness of 8 mm or more, particularly 8 to 25 mm as the hull steel plate of this class from the viewpoint of securing strength. Therefore, in the present invention, the plate thickness of the steel plate is set to 8 mm or more. The upper limit of the plate thickness is determined according to the size of the hull and the like.

[0063]

EXAMPLES Examples of the present invention will be described in comparison with comparative examples.

Using the steel materials having the chemical compositions shown in Table 1, the steel materials were rolled under the manufacturing conditions shown in Table 2 to 12.5 m.
Steel sheets with m and 25.0 mm thickness were produced. And Table 2
Table 2 also shows the structure and material properties of the steel sheet obtained under these manufacturing conditions.

In Table 1, steel types A to F are steels containing components in the range specified in the present invention, and steel types G to I are:
A steel whose composition range is outside the range specified in the present invention.

In Table 2, as-rolled steel sheets No. The developed steels 1 to 5 all have a ferrite space factor of 80% or more within the range specified by the present invention and a ferrite hardness of 160 Hv or more, and the impact absorption energy values are all targeted by the present invention. 120M
It had a value of Pa or more. In addition, steel type F with low Si content
No. using In No. 6, since quenching is performed after the rolling is completed, the ferrite space factor within the range specified by the present invention is 80
% And the ferrite hardness is 160 Hv or more, the impact absorption energy value is 120 which is the target of the present invention.
It had a value of MPa or more. In addition, No. 2 is plate thickness 2
Finished to 5.0 mm.

On the other hand, the comparative steel No. No. 7 used steel type F containing components within the range specified in the present invention, but the Si content was as low as 0.05%, and since the rapid cooling after rolling was not performed, the ferrite space factor was Although it was as high as 85%, the impact energy value was as low as 101 MPa because the ferrite hardness was as low as 140 Hv.
Comparative steel No. In No. 8, steel type G having a C content higher than the range of the present invention was used. Since the Si content was as low as 0.08%, the steel was quenched after the rolling was completed. Therefore, the ferrite hardness is 1
Although it was as high as 70 Hv, the ferrite space factor was as low as 60% and El was also as low as 20%. As a result, TS
Was high, but El could not be secured, and the impact absorption energy value was low at 91 MPa. Comparative steel No. 9 is S
Since the i content was as low as 0.04% and the steel grade H outside the range specified in the present invention was used, the ferrite hardness and the ferrite space factor did not reach the values specified in the present invention. Therefore, the impact absorption energy value was as low as 91 MPa. Comparative steel No. No. 10 had a high C content, which was outside the range of the steel of the present invention, and because the Si content was more than 0.1%, the ferrite hardness was as high as 182, but The rate was as low as 51%, the elongation property was as low as 11%, and the impact energy absorption value was as low as 55 MPa.

As is clear from the above experimental results, the steel composition and the ferrite space factor defined by the present invention are 80% or more,
A steel plate having a ferrite hardness of 160 Hv or higher is 120
It was confirmed that it had a shock absorption energy value of MPa or more.

Further, the steel sheets having the steel components specified in the present invention all have carbon equivalents (Ceq) of 0.42 or less, and as a result of the oblique Y-type weldability test, it is confirmed that all are good. did it.

However, the comparative steel No. No. 8 had a high carbon equivalent (Ceq), and the result of the diagonal Y-type weldability test was poor.

[0071]

[Table 1]

[0072]

[Table 2]

[0073]

According to the present invention, the impact absorption energy (E
A) is capable of providing a steel plate having an excellent impact energy absorption capacity of 120 MPa or more, and by using the steel plate of the present invention for a hull such as a tanker, even if a collision accident occurs between hulls, It is possible to prevent the hull from breaking and opening a hole, or to reduce the breaking area as compared with the conventional steel plate.

Therefore, excellent effects can be obtained from the viewpoint of environmental protection and safety, such as marine pollution due to oil outflow at the time of a tanker collision accident, or reduction of the amount of water flooded from the collision damaged portion.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing destruction of a side surface of a tanker when the tankers collide with each other.

FIG. 2 is a diagram showing a relationship between an elongation property (EL) of a steel material and a steel structure.

FIG. 3 Ferrite hardness (Hv) and strength characteristics (YP + T
It is a figure which shows the relationship with S).

FIG. 4 is a diagram showing a relationship between ferrite hardness (Hv) and elongation property (EL).

FIG. 5: Solid solution Nb amount (wt%) and ferrite hardness (H
It is a figure which shows the relationship with v).

FIG. 6 is a diagram showing changes in shock absorption energy (MPa) with respect to changes in ferrite space factor and ferrite hardness.

[Explanation of symbols]

1 Tanker side wall 2 Tanker bow

Front page continued (72) Inventor Tsuguro Imai 2-6-3 Otemachi, Chiyoda-ku, Tokyo Within Nippon Steel Co., Ltd. (72) Inventor Shuichi Oita-shi, Nishinosu 1 Shin-Nippon Steel Co., Ltd. Oita Works (72) Inventor Masanori Minagawa 1 Nishinosu, Oita-shi, Oita-shi Nippon Steel Co., Ltd. Oita Works (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38 / 60

Claims (5)

(57) [Claims] 1. In weight percent, C: 0.05 to 0.2%, Si: 0.05 to 1.0%, Mn: 0.5-2.0%, Al: 0.001-0.1%, And carbon equivalentCeq (= C + Mn / 6)
(%) Is 0.42 (%) or less, and the balance Fe and
A steel plate with a thickness of 8 mm or more made of inevitable impurities, and
It has a ferrite-based structure with a space factor of 80% or more.
And the hardness of the ferrite phase is Hv 160 or more.
Steel plate for hulls with excellent impact resistance. 2. An unavoidable impurity, S, in weight% is S: ≦.
The steel plate for a hull excellent in impact resistance according to claim 1, wherein the content is 0.005%. 3. Further, in% by weight, Nb: 0.001 to
0.1% is contained, The steel plate for hulls excellent in the impact absorption resistance in any one of Claim 1 and 2 characterized by the above-mentioned. 4. Further, by weight%, V: 0.001-0.1%, Ti: 0.001-0.05%, Ta: 0.001-0.1%, Cr: 0.01- 1.0%, Ni: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 1.0%, and one or more of them are contained , And carbon equivalent Ce
q (= C + Mn / 6 + (Cu + Ni) / 15 + (V + M
o + Cr) / 5) (%) is 0.42 (%) or less, excellent in impact absorption capacity according to any one of claims 1 to 3. Steel plate for hull. 5. Further, in% by weight, Ca: 0.0001
~ 0.01%, Mg: 0.0001-0.01%, RE
M: 0.001-0.05% of 1 type or 2 types or more are contained, The hull excellent in the shock absorption capacity as described in any one of Claim 1 to 4 characterized by the above-mentioned. Steel plate.