JP3434431B2 - Steel plate excellent in impact energy absorbing ability and method for producing the same - 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 material used for a structure such as a shipbuilding, even if a collision accident of a ship, which is represented by an oil spill accident due to a collision of a tanker, should be destroyed. The present invention relates to a steel plate having a high collision energy absorption capability that can be stopped with a minimum difficulty, and a manufacturing method thereof.

[0002]

2. Description of the Related Art In recent years, in the field of shipbuilding, marine pollution caused by oil spills on tankers has become a serious social problem, and even in the unlikely event of a collision between ships, it is difficult to minimize the destruction of the oil. Shipbuilding technology and steel sheets used for it are being studied to minimize damage such as launching from damaged areas.

A typical example of the above-mentioned prior art is the double structuring and legislation of the hull, but in a large tanker, the energy at the time of collision is large and the oil spill is completely suppressed even if the double structuring is adopted. There is also a view that it is difficult.

Therefore, in recent years, as a characteristic of the steel material itself used for the hull, improvement of energy absorption capacity at the time of collision is desired.

However, heretofore, almost no studies have been made from the viewpoint of steel materials regarding the energy absorption capacity at the time of collision of a ship, and in order to improve the energy absorption capacity of steel materials,
It is necessary to study the relationship between the mechanical properties and structure of steel materials and the impact energy absorption capacity.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a steel material having excellent collision energy absorption capability and capable of minimizing damage during collision of a ship, and a method for manufacturing the steel material.

[0007]

[0008]

Means for Solving the Problems (1) The composition of the steel sheet is wt%, C: 0.04 to 0.15% Si: 0.1 to 0.5% Mn: 0.5 to 1.8% Ti: 0.01% or less Al: 0.06% or less Nb: 0.01 to 0.03% and carbon equivalent Ceq (= C + Si / 24 + Mn / 6)
(%) Is 0.4% or less, the balance consists of Fe and inevitable impurities , and the structure of the steel sheet has an odor.
The ferrite fraction F is 80% or more, and
The hardness H of the iron is a relational expression of H ≧ 400-2.6 × F (only
Where H is the Vickers hardness Hv of ferrite and F is
A steel sheet excellent in collision energy absorption capacity, characterized by satisfying a ferrite fraction%) .

( 2 ) In wt%, C: 0.04 to 0.15% Si: 0.1 to 0.5% Mn: 0.5 to 1.8% Ti: 0.01% or less Al: 0 0.06% or less Nb: 0.01 to 0.03% and carbon equivalent Ceq (= C + Si / 24 + Mn / 6)
(%) Is 0.4% or less, and the balance consists of Fe and unavoidable impurities. After heating the steel to 1100 ° C. or higher, it is subjected to hot rolling, and the rolling is finished at 800 ° C. or higher. A relational expression in which the ferrite fraction F is 80% or more, and the hardness H of the ferrite is H ≧ 400-2.6 × F.
(However, H in the formula is Vickers hardness H of ferrite.
v, F manufacturing method of steel sheet excellent in impact energy absorbing capacity to meet the mean ferrite fraction%).

[0010]

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

In order to investigate the relationship between the collision energy absorption capacity of the hull and the mechanical properties and structure of the steel material, an evaluation test of collision energy absorption capacity as shown in FIG. 1 was conducted.

In the test, a large impact tester is used, the end 1 of the test piece is fixed to the tester, and the hammer 2 is made to collide with the central part 3 of the test piece. The width of the test piece is small at the center of the test piece, and the test piece breaks from the center.

The impact energy absorption capacity (EA) is the swing-up angle θ of the hammer when the center portion of the test piece is broken using a hammer having a hammer arm length: hcm and weight: wkg.
Then, the potential energy of the hammer is calculated from the swing-back angle γ of the hammer after breakage of the test piece, and this value is divided by the cross-sectional area of the central portion of the test piece.

In the test, C: 0.04 to 0.2% and Si: 0.1 to 1.0 were investigated in order to investigate the relationship between the collision energy absorption capacity (EA) and each of the mechanical properties and structure of the steel material.
%, Mn: 0.5 to 2.0%, Ti: 0 to 0.02%,
Tests were carried out using test pieces of the same shape with steel materials having various components of Al: 0.06% or less and Nb: 0 to 0.03%, and collision energy absorption capacity (E
The relationship between A) and mechanical properties and steel structure was investigated.

As a result, collision energy absorption capacity (EA)
And the mechanical properties of steel, the elongation properties (E
It was found that both L) and strength properties (YP, TS) are excellent in order to improve the collision energy absorption capacity (EA), and the following correlation was obtained.

EA = α × EL × (YP + TS) / 2 (1) where EL is total elongation (%) and YP is yield strength (kgf
/ Mm ² ), TS is the tensile strength (kgf / mm ² ),
α is a constant determined by the shape of the test piece.

Further, as a result of investigating the relationship between the elongation property (EL) of the steel material and the structure, as shown in FIG.
It was found that L) was good when the ferrite fraction (F) was 80% or more.

On the other hand, the strength characteristics YP and TS tend to generally increase as the second phase structure fraction of bainite, pearlite, etc. other than ferrite increases, but the elongation characteristics (EL) In order to improve it, it is necessary to make the second phase structure fraction as small as possible.

Therefore, while securing EL, YP, TS
As a means of increasing the hardness of the ferrite phase(Vicker
Hardness Hv)Paying attention to (H), the same ferrite fraction%
In case of (F), ferrite hardnessHvCollision due to (H)
To investigate how energy absorption capacity (EA) changes
It was The result is shown in FIG.

The collision energy absorption capacity (EA) of steel materials for hulls that have been generally used in the past is about 10 to 11. The goal of the present invention is the collision energy of this conventional steel plate.
It is to improve the absorption capacity (EA) by at least 20%, that is, to obtain a steel sheet having a collision energy absorption capacity (EA) of 12 or more.

From the test results of FIG. 3, the ferrite fraction (F)
Is 90%, the ferrite hardness (H) is Hv 170 or more, and the ferrite fraction (F) is 95%, the ferrite hardness (H) is Hv 154 or more.

It is also understood that when the ferrite fraction (F) is 80%, the ferrite hardness (H) needs to be Hv 192 or more.

However, the ferrite fraction (F) is 7
At 0%, since the total elongation (EL) is small as shown in FIG. 2, even if the ferrite hardness (H) is increased to about Hv 195, the collision energy absorption capacity (EA) is 12 or less, The target value of the present invention cannot be satisfied.

From the relationship between the collision energy absorption capacity (EA) and the ferrite fraction (F) and the ferrite hardness (H), the critical ferrite fraction (F) and the critical ferrite hardness necessary to achieve EA ≧ 12 are obtained. The relationship (H) is obtained as shown in FIG.

Here, the limit ferrite fraction (F) and the limit ferrite hardness (H) are the ferrite fraction (F) and the limit ferrite hardness (H) required to satisfy EA ≧ 12 in FIG. And the respective limit values.

From this result, the collision energy absorption capacity (E
In order to achieve A) of 12 or more, H ≧ 400−2.6 × F and F ≧ 80 () between the limit required ferrite fraction (F) and the limit required ferrite hardness (H). %) (Was
However, H is the Vickers hardness Hv of the ferrite, and F is the flux.
Elite fraction means%)

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

C is necessary to secure the strength of the steel material, but when the C content is 0.15% or more, it becomes difficult to secure the ferrite fraction of 80% or more, and 0.04%.
In the following, the ferrite hardness does not reach Hv 170 or higher, so the content was defined as 0.04% to 0.15%.

Si is required to be 0.1% or more as a deoxidizing element of steel, but if it exceeds 1%, a serious problem occurs in weldability, so 0.1 to 1% is specified.

Mn secures the toughness of the steel and improves the strength. Therefore, the lower limit is 0.5%. Excessive addition of Mn hinders the securing of the ferrite fraction.
% Was set as the upper limit.

Ti may be added in order to refine the structure and ensure toughness, but Ti easily forms precipitates, and in particular Nb and Ti- which are added to increase the hardness of ferrite. Since Nb composite precipitates are generated, it is harmful to increase the ferrite hardness, so the upper limit was restricted to 0.01%.

Al is added as a deoxidizing element.
If it exceeds 06%, a precipitate is formed, the toughness of the base material becomes brittle, and the toughness of the welded joint is adversely affected. Therefore, the upper limit is set to 0.06%.

Nb is added in an amount of 0.01% or more in order to form a solid solution in the ferrite base or to strengthen the precipitation by fine precipitates, but excessive addition is harmful to the toughness of the welded joint, so the upper limit is 0. Specified as 0.03%.

The carbon equivalent Ceq (= C + Si / 24
If + Mn / 6) (%) is 0.4% or more, the hardenability is increased, the generation of ferrite becomes difficult, and it becomes difficult to secure the ferrite fraction. Therefore, the upper limit is defined as 0.4%. .

Next, the reasons for limiting the production conditions for the steel of the present invention will be described.

In the manufacturing method of the present invention, the heating temperature is
Nb, which is added to increase the ferrite hardness, is used to completely dissolve the Nb in the austenite phase before rolling.
Specify at 100 ° C or higher.

Further, the rolling temperature is defined as 800 ° C. or higher in order to suppress the effect of promoting the precipitation of Nb when processed at a temperature lower than 800 ° C.

[0038]

EXAMPLES Using steel materials having the chemical composition shown in Table 1,
Steel plates were manufactured under the manufacturing conditions shown in Table 2. In addition, Table 2 also shows the structure and mechanical properties of the steel sheet obtained under these conditions.

In Table 1, Steel Grades 1 to 4 are steels containing the components in the range specified in the present invention. On the other hand, the steel types 5 to 7 show examples in which the contents of Ti and Nb both do not satisfy the specified range of the present invention as comparative examples.

In Table 2, Nos. 1 to 7 are examples of the present invention, but the ferrite fraction (F) is 80%, and the hardness (H) of ferrite is 400-2.6 × F or more. And the impact absorption energy (EA) of the steel sheet is 12 kgf / mm ² or more, which is the target of the present invention.

On the other hand, although the chemical compositions of Nos. 8 and 9 are within the scope of the present invention, under the manufacturing conditions, No. 8 has the hardness (H) of the ferrite because the heating temperature and the rolling end temperature are lower than the predetermined temperature. ) Is less than 400-2.6 × F, and the collision absorption energy (EA) is less than 12 kgf / mm ² . No. 9 has a rolling end temperature lower than a predetermined temperature and a sufficient impact absorption energy (EA) of 12 kg.
It is less than f / mm ² .

Nos. 10 to 12 have chemical components outside the scope of the present invention, and No. 10 has ferrite hardness (H).
Has not reached a predetermined value, the number 11 has not obtained a predetermined ferrite fraction (F), and the number 12 has a ferrite hardness (H) that has not reached a predetermined value, so the collision absorption energy (EA) Is less than 12 kgf / mm ² .

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

According to the present invention, the collision absorption energy (E
A) is capable of providing a steel material having an excellent impact energy absorption capacity of 12 kgf / mm ² or more, and by using the steel material of the present invention for a hull, even if a collision accident occurs between hulls, The breaking area of the hull can be reduced as compared with the case of conventional steel materials.

Therefore, there is an immeasurable effect in terms of environmental protection and safety, such as marine pollution due to oil spill at the time of a tanker collision accident, or reduction of the amount of water launched from a collision damaged part.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a collision energy absorption capacity evaluation test method.

FIG. 2 is a diagram showing a relationship between a ferrite fraction (F) and a total elongation (EL).

FIG. 3 is a diagram showing a relationship between ferrite hardness (H) and collision energy absorption capacity (EA).

FIG. 4 is a diagram showing a relationship between a limit required ferrite fraction (F) and a limit ferrite hardness (H).

Front Page Continuation (72) Inventor Shuichi Oita City, Oita City, Nishinosu 1 No. 1 Nippon Steel Corporation Oita Works (56) References JP-A-8-3679 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/02

Claims (2)

(57) [Claims] 1. The composition of the steel sheet is wt%, C: 0.04 to 0.15% Si: 0.1 to 0.5% Mn: 0.5 to 1.8% Ti: 0.01% or less Al: 0.06% or less Nb: 0.01 to 0.03% and carbon equivalent Ceq (= C + Si / 24 + Mn / 6)
(%) Is 0.4% or less, the balance is Fe and unavoidable
Relational expression of the ferrite fraction F of 80% or more and the hardness H of the ferrite H ≧ 400-2.6 × F in the structure of the steel sheet (only
Where H is the Vickers hardness Hv of ferrite and F is
A steel sheet excellent in collision energy absorption capacity, characterized by satisfying a ferrite fraction%) . 2. In % by weight, C: 0.04 to 0.15% Si: 0.1 to 0.5% Mn: 0.5 to 1.8% Ti: 0.01% or less Al: 0. 06% or less Nb: 0.01 to 0.03% and carbon equivalent Ceq (= C + Si / 24 + Mn / 6)
(%) Is 0.4% or less, after the balance was heated steel consisting of Fe and unavoidable impurities than 1100 ° C., thermal
It is used for hot rolling and finished at 800 ℃ or higher.
The ferrite fraction F as a characteristic is 80% or more, and
The relational expression where the hardness H of the ellite is H ≧ 400-2.6 × F
(However, H in the formula is Vickers hardness H of ferrite.
v, F manufacturing method of steel sheet excellent in impact energy absorbing capacity to meet the mean ferrite fraction%).