JP2991586B2 - Steel sheet excellent in fatigue propagation resistance and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is suitable for thick steel plates used for large structures such as ships, bridges, marine structures, etc., which can be broken by fatigue cracks caused by fluctuating loads generated in the structures. The present invention relates to a steel sheet excellent in fatigue crack propagation resistance and a method for manufacturing the same.

[0002]

2. Description of the Related Art When a fluctuating load is continuously applied to a steel sheet, a fatigue crack may occur. Especially in welds,
Since there is a discontinuous portion between the weld metal and the steel sheet, stress tends to concentrate, and it is likely to be a starting point of fatigue cracking. Usually, the fatigue cracks that have occurred continue to propagate, and in the worst case, lead to the destruction of the structure itself. If the structure is a ship, bridge, marine structure, etc., the social impact of destroying it is large,
In many cases, it is expected that there is a danger of human life.

[0003] In these structures, a design is made so that stress concentration does not occur structurally so that fatigue fracture does not easily occur at a welded portion, or a shape with a particularly careful shape is formed at a boundary between a weld metal and a steel plate. At present, welding is performed to avoid stress concentration. Therefore, in these methods, efficient design cannot be performed due to great restrictions on the design, and it takes a long time to finish the welding, which is inefficient and at the same time causes high cost.

[0004]

In ships, bridges, offshore structures and the like, thick steel plates having a thickness of 6 mm or more are usually used, and are assembled by fusion welding. As mentioned above,
It is very difficult to avoid the generation of fatigue cracks because stress concentration is likely to occur in a weld. However, even if a fatigue crack occurs, if the fatigue propagation speed of the steel sheet is low, the crack can be found and repaired by periodic inspections or the like before the structure is destroyed. Therefore, if the steel sheet can be provided with fatigue propagation suppression characteristics, the above-mentioned problem can be solved.

As a method focused on improving the fatigue crack propagation resistance of a steel sheet, there is a method disclosed in Japanese Patent Application Laid-Open No. 3-291355. Japanese Patent Application Laid-Open No. Hei 3-291355 proposes that a monotonous and continuous strength gradient be provided in the thickness direction. Therefore, an object of the present invention is to provide a steel sheet itself with fatigue propagation suppression characteristics by a method different from that of Japanese Patent Application Laid-Open No. 3-291355.

[0006]

Means for Solving the Problems The present inventors have conducted various studies to solve the above-mentioned problems. As a result, the structure is one or two of ferrite, pearlite, and bainite.
Mainly consists of a species or more, the steel sheet further average presence spacing 20μm or less island martensite and has a mean flatness ratio 5 or more shapes is present in a proportion of 0.5% to 5% in terms factor fatigue Was found to be slow.

The gist of the steel sheet of the present invention has the following constitution. (1) By weight%, C: 0.03 to 0.20%, Si: ≤ 0.50%, Mn: 0.4 to 1.6%, P: ≤ 0.020%, S: ≤ 0,0%. 010%, Al: ≦ 0.10% , N: ≦ 0.006%, the balance of steel consisting of iron or unavoidable impurities, yet set
Weave is one of ferrite, pearlite, bainite or
It is mainly composed of two or more kinds, and the average existence interval is 20 μm
Island-shaped marten with a shape of less than or equal to 5 and an average aspect ratio of 5 or more
Sites are present at a rate of 0.5-5% in area ratio
Steel sheet with excellent fatigue propagation resistance characterized by (2) The above steel is further expressed in weight% : Cu: ≦ 1.0%, Ni: ≦ 1.5%, Nb: ≦ 0.1%, V: ≦ 0.1%, Cr: ≦ 1.0% , Ti: 0.005 to 0.02%, B: 0.0005 to 0.0020%, characterized by containing one or more of them.
The steel sheet excellent in fatigue propagation resistance according to the above (1).

The gist of the method for producing the steel sheet of the present invention is as follows.
It consists of. (3) By weight%, C: 0.03 to 0.20%, Si: ≤ 0.50%, Mn: 0.4 to 1.6%, P: ≤ 0.020%, S: ≤ 0. 010%, Al: ≤ 0.10%, N: ≤ 0.006%, balance: iron or steel consisting of unavoidable impurities, made into a slab by continuous casting or ingot casting, rolled directly or rolled after reheating and finished Ar ₃ points in rolling + 10 ° C. to Ar ₃
Rolling is completed within the range of the point -50 ° C, and then Ar ₃ point -5
Starting water cooling from the temperature of 0 ° C or less at the same time on both sides of the steel sheet,
Air cooling after stopping water cooling at 600 ° C. below the temperature, yet
The structure is one of ferrite, pearlite, bainite or
Is mainly composed of two or more kinds,
m or less and an average aspect ratio of 5 or more
Site is present at a rate of 0.5 to 5% in area ratio.
For producing steel sheets with excellent fatigue propagation resistance characterized by
Law. (4) The above slab is further expressed by weight% : Cu: ≤ 1.0%, Ni: ≤ 1.5%, Nb: ≤ 0.1%, V: ≤ 0.1%, Cr: ≤ 1.0% , Ti: 0.005 to 0.02%, B: 0.0005 to 0.0020%, characterized by containing one or more of them.
Method for producing a steel sheet having excellent fatigue propagation resistance according to (3) above
Law.

The reasons for limiting each component are described below. The reasons for limiting the components are the same as the reasons for limiting the components in general structural steel. C is necessary for increasing strength and for producing island-like martensite. However, if it is too small, it has no effect, and if it is too large, it causes a decrease in weldability and a decrease in toughness of a base material and a welded portion. 03% and the upper limit was 0.20%.

[0010] Si is effective for deoxidizing molten steel, but too much Si causes a decrease in weldability and a decrease in toughness of the base metal and the welded portion. Mn, like C, is necessary for increasing strength and forming island-like martensite. However, if it is too small, it has no effect, and if it is too large, it causes a decrease in weldability and a decrease in toughness of a base material and a weld. Was 0.4% and the upper limit was 1.6%. It is desirable that P is low for improving the properties of the base material, but lowering P causes an increase in cost. Therefore, the upper limit is set to 0.02% in terms of economy. S, like P, is desirably low to improve the properties of the base material, but lowering S causes an increase in cost. Therefore, the upper limit is set to 0.01% from the viewpoint of economy.

[0011] Al is essential for deoxidation, but if it is too much, inclusions increase, which is detected as a defect in ultrasonic flaw detection of the steel sheet or causes a decrease in the toughness of the base material. 0.1%. N has an upper limit of 0. 0 if the amount of solid solution increases, the toughness of the heat affected zone significantly decreases.
006%. Cu can increase the strength without deteriorating the toughness of the base material, but if too much, the toughness of the welded portion is reduced. Therefore, the upper limit was set to 1.0%.

[0012] Ni can increase the strength without impairing the toughness of the base material, but if too much, Ni reduces the toughness of the welded portion.

[0013] Nb can improve the toughness of the base material and at the same time increase the strength, but if too much, Nb lowers the toughness of the welded portion, so the upper limit is 0.1%. V is
Although the strength can be increased without impairing the toughness of the base material, if too large, the toughness of the welded portion is reduced.
% As the upper limit. Cr can increase the strength without deteriorating the toughness of the base material, but if too much, lowers the toughness of the base material and the welded portion. Therefore, the upper limit was made 1.0%.

Ti can improve toughness by generating TiN in the steel to suppress the growth of austenite grains at the time of heating during reheating rolling and in the weld heat affected zone. However, if the addition amount is too small, the effect is not obtained. If the addition amount is too large, the toughness of the base material and the weld heat affected zone is significantly reduced due to the generation of TiC.
% And the upper limit is set to 0.02%. B is added as solid solution B to increase the strength of the base material and to precipitate BN and Fe ₂₃ CB ₆ to refine the structure in the weld heat affected zone. If the amount is too small, the effect is not obtained. If the amount is too large, the weldability and the toughness are reduced. Therefore, the lower limit is 0.0005% and the upper limit is 0.00.
20%.

In direct rolling or reheating rolling of the slab having the above-mentioned components, rolling is finished in the range of Ar ₃ points + 10 ° C. to Ar ₃ points−50 ° C. in finish rolling, and then Ar ₃ points−50 ° C. or less By starting water cooling at the same time on both sides of the steel sheet from the temperature of the steel sheet, by stopping the water cooling at a temperature of 600 ° C. or less and then air cooling, the structure is mainly composed of one or more of ferrite, pearlite, bainite,
Further Mean presence spacing 20μm or less and island martensite in which the average flatness ratio of 5 or more shape, it is possible to manufacture a steel sheet that is present in a proportion of 5% or less surface factor, can reduce the growth rate of fatigue cracks . However island martensite, when large quantities are present, the surface factor of the upper limit for significantly reducing the toughness of Charpy test values such as 5%, the surface factor of the lower limit since it is not too small effect 0.5 %.

[0016]

The present inventors have investigated the fatigue propagation speed of several types of steel plates manufactured from slabs having the following components, obtained the results shown in FIG. 3, and found the following. The lower the finishing temperature, the slower the rate of fatigue crack propagation. The water-cooled steel sheet has a lower fatigue crack propagation speed than the air-cooled steel sheet and the water-cooled steel sheet after rolling.

[0017]

[Table 1]

As a result of examining the structure of the fatigue crack propagation portion of these steel sheets, it was found that in the steel sheet having a low finish rolling end temperature and water cooling after the rolling, island martensite was formed in the steel sheet, and moreover, island martensite was formed. It was discovered that fatigue crack branching was observed at the site formation site. It is predicted that when the fatigue crack branches, the stress at the tip of the crack decreases, and it is considered that the propagation speed of the fatigue crack decreases due to the decrease in the stress.

Since the water cooling is performed simultaneously on the front and back surfaces of the steel sheet, the strength (hardness) of the steel sheet is
As shown in FIG. 4, it does not have a monotonous and continuous intensity gradient in the thickness direction as disclosed in Japanese Patent Application Laid-Open No. 3-291355.

Therefore, as a result of further investigation of the organization,
It has been found that when the island martensite is dispersed in a layered structure in a structure mainly composed of ferrite, pearlite or bainite, and the average interval thereof is 20 μm, the above-mentioned delay in the fatigue crack propagation speed occurs. Furthermore, in order for fatigue cracks to branch, the shape of the island-like martensite extends parallel to the surface of the steel sheet, and the average of the flatness ratio (length of major axis / length of minor axis) is 5 or more. I found something better.

Further, as a result of studying the manufacturing conditions for obtaining the above structure, the components may be the same as those of ordinary structural steels.
was flattened by the austenite grains by the r ₃ point + 10 ° C. to Ar ₃ point -50 ° C., then concentrated ingredients into austenite by starting the advanced Ar ₃ point -50 ° C. water cooling from the temperature of the ferrite transformation The cooling rate was increased by cooling with water to transform the austenite in which the components were concentrated into island martensite, and at 600 ° C.
It has been found that it is necessary to stop the water-cooling at the following temperature and then air-cool to leave the island-like martensite after transformation.

[0022]

EXAMPLES Tables 2 and 3 show examples of the present invention. In order to determine the main rolling conditions, it is necessary to determine the Ar ₃ point, which was determined by calculation using the following equation. Ar ₃ = 1108-777 × C-200 × Mn-23 ×
Cu-38 × Ni

The fatigue propagation speed is obtained by applying a stress to the test piece having the shape shown in FIG. 1 by the three-point bending method shown in FIG. 2 and reducing the stress amplitude at every given number of stress amplitude applications. And the value obtained by dividing the fatigue crack growth length between the beach marks by the number of stress amplitude additions was used as an index.
This value indicates the propagation length of the fatigue crack per one stress amplitude, and the smaller the value, the slower the propagation of the fatigue crack. The specimen was sampled so that the major axis direction was parallel to the rolling direction.

The main structure of the steel material is that a small specimen cut out from the steel material is polished in a section in the thickness direction (a plane parallel to the rolling direction), and is polished to a mirror-finished surface with a 5% nitric acid-alcohol solution. Corrosion was performed and the surface was determined by observation with a metallurgical microscope.

The island-like martensite in the steel material is obtained by polishing a cross section in a thickness direction (a surface parallel to the rolling direction) of a small specimen cut out from the steel material and polishing it to a mirror-finished surface with 1% sodium pyrosulfite-4%. % Picric acid by corrosion in a mixed solution. The resulting island-like martensite was photographed, and the photograph was analyzed by an image analyzer to determine the area ratio , average existence interval, and average aspect ratio.

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

No. A, C, F, H, J, and L are the steels of the present invention. B, D,
Compared with E, G, I, K, and M, the steel of the present invention has a fatigue propagation speed reduced to 1/3 to 1/4, and the effect of the present invention is recognized.

[0030]

By using the steel sheet of the present invention for a structure where a fatigue crack may occur, the growth of the fatigue crack can be delayed, the fatigue design conditions can be relaxed, the period of the periodic inspection can be extended, The effect of avoiding the destruction of the structure can be expected, and the social significance of the present invention is very large.

[Brief description of the drawings]

1 (a), 1 (b) and 1 (c) show explanatory views of the shape of a test piece used for investigating fatigue propagation characteristics.

FIG. 2 is an explanatory view of a method for applying stress to a test piece used for investigating fatigue propagation characteristics.

FIG. 3 is a table showing the relationship between the rolling end temperature, the presence or absence of water cooling after rolling, and the fatigue crack propagation speed.

FIG. 4 is a table showing the hardness distribution of the steel of the present invention in the thickness direction.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yuji Funatsu 1 Nishinoshima, Oita, Nippon Steel Corporation Oita Works (56) References JP-A-59-110729 (JP, A) Akira Tokubo 62-35452 (JP, B2) JP-B-60-9086 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00-38/54 C21D 8/02

Claims (4)

(57) [Claims] 1. A weight%, C: 0.03~0.20%, Si : ≦ 0.50%, Mn: 0.4~1.6%, P: ≦ 0.020%, S: ≦ 0.010%, Al: ≤ 0.10%, N: ≤ 0.006%, balance being steel consisting of iron or unavoidable impurities, and one or more of ferrite, pearlite, bainite microstructures And the average existence interval is 20 μm
A steel sheet having excellent fatigue propagation resistance, characterized in that island martensite having a shape of not more than 5 and having an average aspect ratio of 5 or more is present in an area ratio of 0.5 to 5%. 2. The steel according to claim 1, further comprising : by weight : Cu: ≦ 1.0%, Ni: ≦ 1.5%, Nb: ≦ 0.1%, V: ≦ 0.1%, Cr: ≦ 1. 0%, Ti: 0.005 to 0.02%, B: 0.0005 to 0.0020%, characterized by containing one or more of them.
The steel sheet having excellent fatigue propagation resistance according to claim 1. 3. In% by weight, C: 0.03 to 0.20%, Si: ≤ 0.50%, Mn: 0.4 to 1.6%, P: ≤ 0.020%, S: ≤ 0.010%, Al: ≤ 0.10%, N: ≤ 0.006%, with the balance being steel or iron or unavoidable impurities.
Or slab by agglomeration, pressure after direct rolling or reheating
And then, in finish rolling, Ar ₃ points + 10 ° C. to Ar ₃
Rolling is completed within the range of the point -50 ° C, and then Ar ₃ point -5
Starting water cooling from the temperature of 0 ° C or less at the same time on both sides of the steel sheet,
After stopping water cooling at a temperature of 600 ° C or less, air-cool,
The structure is one of ferrite, pearlite, bainite or
Is mainly composed of two or more kinds,
m or less and an average aspect ratio of 5 or more
A method for producing a steel sheet having excellent fatigue propagation resistance, characterized in that insites are present in an area ratio of 0.5 to 5% . 4. The slab further comprises, by weight : Cu: ≦ 1.0%, Ni: ≦ 1.5%, Nb: ≦ 0.1%, V: ≦ 0.1%, Cr: ≦ 1. 0%, Ti: 0.005 to 0.02%, B: 0.0005 to 0.0020%, characterized by containing one or more of them.
4. A method for producing a steel sheet having excellent fatigue propagation resistance according to claim 3.
Law.