JP2967889B2 - Method for producing steel with excellent fatigue crack growth characteristics in wet hydrogen sulfide environment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing steel excellent in fatigue crack growth characteristics in a wet hydrogen sulfide environment suitable as a steel plate for a crude oil tanker used for transporting crude oil containing hydrogen sulfide.

[0002]

2. Description of the Related Art It is already well known that hydrogen-induced cracking (HIC) or sulfide stress cracking (SSC) poses a problem in steel materials used for the above applications.
Numerous studies have been made on the prevention, and many countermeasures have been proposed.

[0003] HIC is a crack generated in a steel material without external stress, and SSC is a crack under static stress.
HIC and SSC are hydrogen embrittlement caused by the penetration of hydrogen generated when steel corrodes in a wet hydrogen sulfide environment into steel, and is one of the embrittlement phenomena of steel.

[0004] On the other hand, fatigue fracture and corrosion fatigue fracture that occur under the condition of repeated stress are another major embrittlement phenomenon of steel.

[0005] There have also been many studies on fatigue and corrosion fatigue of marine or marine structures, automobile wheels and crankshafts, and gear materials, etc., which are subjected to repeated stresses due to waves.

Recently, high-strength steel has been widely used in crude oil tankers and the like from the viewpoint of increasing the size and cost. In this case, the steel material is subjected to more stress than before, and there has been a concern about fatigue other than cracking. However, there are few examples of investigating the fatigue behavior of a steel used in a crude oil tanker, a line pipe, a petroleum refining apparatus, and the like exposed to a wet hydrogen sulfide environment as described above in a wet hydrogen sulfide environment.

[0007] Corrosion NACE, vol.
32, no. 12 (December) 1976). "Fatigue" by VOSHKOVSKI
Crack Growth in an X65 Li
ne-Pipe Steelin Sour Cloud
The report entitled “e Oil” shows that the fatigue crack growth rate is significantly accelerated when the concentration of hydrogen sulfide increases, and that the environmental effects on fatigue in a wet hydrogen sulfide environment cannot be ignored. Conceivable. It is considered that the acceleration of the crack growth rate presumed to be caused by hydrogen sulfide is superimposed on hydrogen embrittlement. However,
There are few examples of detailed studies on the material factors that affect fatigue in a wet hydrogen sulfide environment, and no specific countermeasures have been found.

FIG. 1 shows the results of a fatigue test of conventional steel in air and in a wet hydrogen sulfide environment. Da / dN on the vertical axis
Is a crack growth rate, which is represented by a growth distance (mm) per one stress cycle. ΔK on the horizontal axis is the difference between the maximum stress intensity factor (Kmax) and the minimum stress intensity factor (Kmin). That is, ΔK = Kmax−Kmin. In general,
The crack growth rate (da / dN) is expressed as da / d with respect to ΔK.
Since the relationship of N = C (ΔK) m is widely recognized, ΔK is plotted on the horizontal axis. The stress intensity factor is the size and shape of a crack when a crack (defect or crack) is present,
It is treated as stress in fracture mechanics, which standardizes mechanical boundary conditions such as dimensions and shapes of materials and load conditions. The stress intensity factor is also useful when dealing with fatigue crack growth standardized.

As apparent from FIG. 1, the crack growth rate is higher in a wet hydrogen sulfide environment than in the atmosphere. In particular, ΔK
The crack growth is accelerated in a region having a large value (about 20 ksi (in) ^1/2 or more).

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to cope with fatigue fracture in a wet hydrogen sulfide environment in a crude oil tanker, which is expected to frequently occur with the strengthening of steel materials. Things.

A specific object of the present invention is to provide a YS (yield stress) of 20 to 45 kgf / mm used for a crude oil tanker.
2. To provide a method for producing a steel material having a TS (tensile strength) of 35 to 60 kgf / mm2 and having a property that a fatigue crack does not easily propagate when used in a wet hydrogen sulfide environment. is there. More specifically, HIC or SSC does not occur even if conventional ordinary steel or high-tensile steel is used, but in a wet hydrogen sulfide environment where the fatigue crack growth rate is accelerated using conventional high-tensile steel. It is an object of the present invention to provide a method for producing a crude oil tanker steel having excellent fatigue crack propagation resistance.

[0012]

The gist of the present invention resides in the following manufacturing method.

(1) C: 0.01-0.2% by weight
%, Si: 0.2 to 0.6%, Mn: 0.3 to 2.0
%, Sol. Al: contains 0.01 to 0.1%, the remainder is inevitable impurities (however, excluding 0.003% or less of S) and a continuous cast slab or ingot made of Fe and hot forged or hot rolled, Reheat to a temperature range of (Ar 3 points -30 ° C.) or more, accelerate cooling from (Ar 3 points -30 ° C.) or more to a temperature range of 400 to 600 ° C. at a cooling rate of 5 to 25 ° C./s, and then allow to cool or A method for producing a crude oil tanker steel having excellent fatigue crack growth resistance in a slowly cooled wet hydrogen sulfide environment.

(2) In addition to the alloy components described in the above (1), Nb: 0.01-0.1%, Ti:
A continuous cast slab or ingot containing at least one of 0.01 to 0.1% and V: 0.01 to 0.1%, with the balance being unavoidable impurities and Fe, is treated in the same step as (1). For producing crude oil tanker steel having excellent fatigue crack propagation resistance in a wet hydrogen sulfide environment.

(3) In addition to the alloy components described in the above (1), further, by weight%, Cu: 0.1 to 1.0% and Cr:
Fatigue crack growth resistance in a wet hydrogen sulfide environment in which a continuous cast slab or ingot containing 0.1 to 2.0% of one or two kinds and the balance is composed of unavoidable impurities and Fe is treated in the same step as (1). Method for producing crude oil tanker steel.

(4) In addition to the alloy components described in the above (1), further, by weight%, Cu: 0.1 to 1.0% and Cr:
0.1 to 2.0% of one or two, and Nb:
Continuous cast slab containing at least one of 0.01 to 0.1%, Ti: 0.01 to 0.1% and V: 0.01 to 0.1%, with the balance being unavoidable impurities and Fe Alternatively, a method for producing a crude oil tanker steel having excellent fatigue crack propagation resistance in a wet hydrogen sulfide environment in which an ingot is treated in the same step as (1).

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has been made based on the results of a study on material factors affecting the fatigue behavior in a wet hydrogen sulfide environment, which has hardly been studied hitherto.

As will be apparent from the examples below, when the fatigue behavior in a wet hydrogen sulfide environment is observed, crack growth is remarkably accelerated in a stress state where ΔK is large, that is, the plastic deformation region is widened. In fatigue in a wet hydrogen sulfide environment, the crack growth increases as the hydrogen sulfide concentration increases. It is considered that the higher the concentration of hydrogen sulfide, the greater the amount of hydrogen atoms generated due to corrosion that enter the steel and form a solid solution. Further, since crack growth is accelerated in a stress state in which the plastic deformation region is widened, it is presumed that hydrogen embrittlement is greatly involved in this fatigue fracture behavior.

Furthermore, focusing on the fact that this fatigue fracture mainly propagates in the ferrite grains, and that the crack growth rate is high in a region where the plastic deformation is large, the plastic deformation behavior of the ferrite grains is closely related to the fatigue fracture behavior. It is thought that it is.

According to the present invention, if the chemical composition of steel and the manufacturing conditions are properly selected, the strength of ferrite grains can be increased, and the plastic deformation of ferrite grains under repeated stress can be reduced as compared with conventional steels. This is based on the new finding that the growth rate of fatigue cracks can be reduced.

In terms of chemical composition, the addition of one or more of Nb, Ti and V can refine the ferrite crystal grains and strengthen the ferrite itself by the formation of carbides. Can be further reduced. Also, by adding one or both of Cu and Cr, the corrosion resistance of the steel in an atmosphere containing hydrogen sulfide is improved, and the corrosion rate is reduced. As a result, the amount of solid solution hydrogen in the steel is reduced, and the crack growth rate is reduced. Can be smaller.

Hereinafter, the reasons for selecting the chemical composition of the steel used as the material of the production method of the present invention and the reasons for selecting the production conditions will be described in detail.

1) Chemical composition of steel C is a component that increases the strength of steel. In the present invention, in order to maintain the above strength level, the content of C is set to 0.01%.
(Hereinafter,% relating to alloy components means weight%). If it is less than this, it is difficult to secure the strength of the steel required for use of the steel produced by the method of the present invention (hereinafter referred to as the present invention steel for convenience). On the other hand, in order to maintain good weldability of steel, the upper limit of the C content is set to 0.2%. The main use of the steel according to the invention is often subjected to welding. C
If the content is more than 0.2%, so-called weld cracks are liable to occur, that is, cracks occur a short time after welding in the hardened portion of the weld heat affected zone due to hydrogen dissolved in the steel during welding. Welding cracks are greatly affected by the hardness of the heat affected zone, and the harder they are, the more likely they are to crack. Since a material having a higher C is easily cured, a lower C is better to prevent this. Desirable C content is 0.03-0.18%.

[0024] Si is useful as a deoxidizing agent for steel, and is also actively used in the present invention for improving fatigue crack growth characteristics in a wet hydrogen sulfide environment. If the Si content is less than 0.2%, these effects cannot be expected. On the other hand, if Si exceeds 0.6%, the toughness of the steel is impaired.
Desirable content of Si is 0.25 to 0.5%.

Mn is also a component that improves the strength of steel. If it is less than 0.3%, it is difficult to secure the strength required for the intended use of the steel of the present invention. However, Mn, like C, is a component that hardens the weld heat affected zone and induces welding cracks, and therefore has an upper limit on its content. That is, if it exceeds 2.0%, welding cracks are likely to occur. Desirable Mn content is 0.5 to 1.8%.

Al is used for deoxidizing steel, and sol. It is necessary to add Al so as to have a content of 0.01%. However, sol. If the Al content exceeds 0.1%, the cleanliness and toughness of the steel are impaired. Desirable sol. The range of the Al content is 0.01 to
0.05%.

One of the steels used as the raw material of the method of the present invention comprises the above components and the balance of Fe and inevitable impurities. Among impurities, P and S are desirably suppressed to 0.025% or less and 0.020% or less, respectively.

However, it is not necessary to reduce S to 0.003% or less at the expense of refining costs. In a crude oil tanker, HIC and S can be obtained even if S is not suppressed to 0.003% or less.
This is because SC does not occur. This is because even if conventional steel or high-tensile steel is used for crude oil tankers, HIC or SSC
It is also evident from the fact that no was generated. Therefore, it is needless to say that Ca need not be included for sulfide morphology control. That is, in the part where the steel produced by the method of the present invention is used, the HIC resistance and the SS resistance
There is no need to use expensive steel that takes measures against C. When the Ca treatment is performed, a coarse compound compound of Ca sulfide and oxide is generated depending on the treatment conditions, and the fatigue strength may be lowered instead.

The steel used as the material of the method of the present invention may further contain one or more of the following two elements in addition to the above components.

First group: 0.01 to 0.1% of Nb and Ti respectively
And V 2nd group: 0.1-1.0% Cu and 0.1-
2.0% Cr element of the first group has the effect of improving the crack growth characteristics by refining and strengthening the ferrite grains.

If all of Nb, Ti and V are less than 0.01%, the effect of improving the fatigue crack growth characteristics in a wet hydrogen sulfide environment is poor. On the other hand, if the content of each exceeds 0.1%, not only the effect is saturated, but also the strength of the steel becomes too high and the toughness is impaired. In any case, the desirable content is 0.02-
0.05%.

The elements of the second group improve the corrosion resistance of the steel as described above. However, if Cu is less than 0.1%, the effect of addition is small. On the other hand, Cu induces welding cracks and melts at a high temperature to lower the grain boundary strength of the steel to easily generate cracks and scratches during hot rolling, so the content is limited to 1.0%. Should. Desirable Cu content is 0.2 to 0.5%.

[0033] Cr at a content of 0.1% or more is effective for further improving the fatigue crack growth characteristics in a wet hydrogen sulfide environment. However, like Cr and Mn, Cr is a component that hardens the weld heat affected zone and causes welding cracks. Therefore, when Cr is added, the upper limit of the content should be 2.0%. The desirable content of Cr is 0.5 to 1.5%.

One or more elements can be selected from one or both of the first and second groups and added.

2) Manufacturing Method Steel having the above-mentioned chemical composition is prepared by ordinary smelting and casting (continuous casting or ingot casting), followed by hot forging or hot rolling (hereinafter collectively referred to as hot working). Being, boards, tubes,
Products such as flanges.

There is no particular limitation on the heating temperature when hot working a continuously cast slab or ingot. What is necessary is just to heat to the temperature range in which the next hot forging or hot rolling is possible. The finishing temperature of the hot forging or hot rolling is preferably lower than (Ar3-30 ° C), but need not be lower than (Ar3-30 ° C). Thereafter, reheating to (Ar3-30 ° C.) or more is performed, and the cooling is started after the temperature is increased to the finishing temperature or more. At the time of finishing, the transformation is sufficiently advanced, then reheated and the transformation is repeated to make the structure finer throughout the steel material.

What is important is the cooling condition after hot working.
This cooling must be accelerated cooling from a temperature of (Ar 3 point−30 ° C.) or higher. The cooling start temperature is (Ar3-
If the temperature is lower than 30 ° C.), a large amount of pro-eutectoid ferrite is formed, and the ferrite is not strengthened by accelerated cooling and has no effect of improving fatigue crack growth characteristics. Progress is rather accelerated. Desirable cooling start temperature is a temperature of Ar 3 or more.

The cooling rate is in the range from 5 ° C./s to 25 ° C./s. At a cooling rate lower than 5 ° C./s, there is no effect of accelerated cooling, and there is no effect of improving fatigue crack growth characteristics in a wet hydrogen sulfide environment, which is the object of the present invention. On the other hand, 25 ° C /
If the cooling rate exceeds s, not only the strength of the steel becomes too high, but also the above-mentioned block-like bainite and martensite are formed, and the crack growth is accelerated.

The stop temperature of the above-mentioned accelerated cooling is 400 to 6
A range of 00 ° C is appropriate. At temperatures lower than 400 ° C,
As in the case where the cooling rate is too high, the strength of the steel becomes too high, and the formation of block-like bainite and martensite accelerates the crack growth. If the stop temperature of the accelerated cooling exceeds 600 ° C., the ferrite is not strengthened and the effect of the accelerated cooling is poor. After the completion of the accelerated cooling, cooling or slow cooling is performed. Steel material (product) obtained in this way
Can be used as is.

[0040]

The present invention will be described below with reference to examples.

Table 1 shows the chemical composition of the test steel. These steels were melted and continuously cast into 240mm thick slabs.
This was heated to 1150 ° C. and rolled into a 15 mm thick plate.

[0042]

[Table 1]

The finishing temperature was lower than (Ar 3 point-30 ° C.), and after reheating to the temperature shown in Table 2, cooling was performed from the accelerated cooling start temperature also shown in Table 2. Table 2 shows the cooling rate and the accelerated cooling stop temperature.

[0044]

[Table 2]

From the steel sheet obtained as described above, a test piece shown in FIG. 4 (b) was sampled and subjected to a fatigue test in a wet hydrogen sulfide environment using an apparatus shown in FIG. 4 (a). That is, as shown in FIG. 4A, the test piece 1 was repeatedly subjected to stress by the hydraulic cylinder 5 in the test solution tank 2. 3 is a solution circulation pump, 4 is a load cell, 6 is a hydraulic pressure source, 7 is a servo valve, 8 is a waveform generator, and 9 is a load controller.
The fatigue test conditions are as follows.

F (repetition rate) = 30 Hz R (stress ratio) = 0.1 T (test temperature) = room temperature Table 2 shows that in a wet hydrogen sulfide environment, ΔK = 20 ksi (i
n) The ratio of the crack growth rate (da / dN) at ^1/2 and its value to the crack growth rate (da / dN) at ΔK = 20 ksi (in) ^1/2 in the atmosphere. In addition,
The reason why the value at ΔK = 20 ksi (in) ^1/2 was adopted as the crack growth rate is that, as shown in FIG. 1 described below, the crack growth rate greatly differs from that in the atmosphere in this region. It is. 10% water for wet hydrogen sulfide environment
The suspended crude oil containing hydrogen sulfide concentration of 1 vol. % (The remainder is nitrogen) is continuously blown during the test.

FIG. 1 shows a comparison of the crack growth rates of the carbon steel (conventional example) that has been cooled after rolling, in the atmosphere and in a wet hydrogen sulfide environment. As is clear from FIG. 1, the crack growth rate is higher in a wet hydrogen sulfide environment than in the atmosphere. In particular, ΔK
The crack growth in a wet hydrogen sulfide environment is accelerated in a region having a large value (about 20 ksi (in) ^1/2 or more).

FIG. 2 shows the results of investigating the effect of the concentration of hydrogen dissolved in the steel on the fatigue crack growth rate using a carbon steel (conventional example) which was also cooled after rolling. In this experiment, a hydrogen sulfide concentration of 1 vol. % And 10 vol. % Mixed gas and pure hydrogen sulfide gas were used to vary the concentration of dissolved hydrogen. The higher the concentration of hydrogen sulfide, the higher the concentration of hydrogen dissolved in steel. Therefore, as is clear from FIG. 2, the higher the concentration of hydrogen sulfide, that is, the higher the concentration of solid solution hydrogen, the higher the crack growth rate.

FIG. 3 shows the effect of the accelerated cooling stop temperature on the ratio of the crack growth rate in a wet hydrogen sulfide environment to that in the atmosphere.
In the range of 400 ° C., the crack growth rate is suppressed to within three times that in the atmosphere. Although the steel used in FIG. 3 was not reheated, the effect of the accelerated cooling stop temperature was (Ar
The same applies to the case of accelerated cooling by reheating above 3-30 ° C).

Test numbers A-1, B-1, and C-1 in Table 2
And D-1 are examples in which a steel to which neither the first group element nor the second group element was added was used as a raw material, and no accelerated cooling was performed. The crack growth rate in the wet hydrogen sulfide environment of the steel obtained in these examples is more than 5 times that in the atmosphere. On the other hand, the crack growth rate in the wet hydrogen sulfide environment of sample No. L-1 which satisfies all the conditions defined in the present invention was significantly improved to within 2 to 3 times that of the atmosphere. However, even when Nb and Ti were contained, no improvement was observed when the accelerated cooling stop temperature was low as in test sample L-2.

[0051]

As shown in the Examples, the steel produced by the method of the present invention has a remarkably low fatigue crack growth rate in a wet hydrogen sulfide environment, and has a fatigue crack growth rate of 2% in the atmosphere.
Fits within ~ 3 times. That is, this steel has remarkably excellent crack growth characteristics, is exposed to a wet hydrogen sulfide environment,
It is extremely practical as a material for crude oil tankers that are subjected to repeated stress.

[Brief description of the drawings]

FIG. 1 shows the results of a fatigue test of a conventional steel in air and in a wet hydrogen sulfide environment.

FIG. 2 is a graph showing the effect of hydrogen sulfide concentration on the fatigue crack growth rate of conventional steel.

FIG. 3 shows a wet hydrogen sulfide environment (hydrogen sulfide concentration of 1 vol.
FIG. 3 is a graph showing the effect of the accelerated cooling stop temperature after hot rolling on the ratio of the crack growth rate to the crack growth rate in the atmosphere in%).

FIG. 4A is a diagram showing an outline of a fatigue test apparatus, and FIG.
FIG. 3 is a view showing the shape and dimensions of a test piece.

Continuation of front page (72) Inventor Hideaki Yuki 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. (56) References JP-A-2-263918 (JP, A) JP-A-63 JP-A-38519 (JP, A) JP-A-58-77530 (JP, A) JP-A-57-85928 (JP, A)

Claims (4)

(57) [Claims] (1) C: 0.01 to 0.2% by weight, S:
i: 0.2 to 0.6%, Mn: 0.3 to 2.0%, so
l. Al: contains 0.01 to 0.1%, the remainder is inevitable impurities (however, excluding 0.003% or less of S) and a continuous cast slab or ingot made of Fe and hot forged or hot rolled, Reheat to a temperature range of (Ar 3 points -30 ° C.) or higher, and a cooling rate of
A method for producing a crude oil tanker steel having excellent fatigue crack propagation resistance in a wet hydrogen sulfide environment, wherein the steel is acceleratedly cooled at a rate of 25 ° C./s to a temperature range of 400 to 600 ° C., and then cooled or gradually cooled. 2. C: 0.01-0.2% by weight, S
i: 0.2 to 0.6%, Mn: 0.3 to 2.0%, so
l. Al: 0.01 to 0.1%, and further Nb:
0.01 to 0.1%, one or more of Ti: 0.01 to 0.1%, and V: 0.01 to 0.1%, and the remainder is inevitable impurities (however, S.O. (Excluding 003% or less) and a continuous cast slab or ingot made of Fe and hot-rolled or hot-rolled, and then reheated to a temperature range of (Ar 3 point-30 ° C.) or more (Ar 3 point-30 ° C.) or more. A crude oil tanker steel excellent in fatigue crack propagation resistance in a wet hydrogen sulfide environment characterized by accelerated cooling to a temperature range of 400 to 600 ° C. at a cooling rate of 5 to 25 ° C./s and then allowing it to be cooled or gradually cooled. Production method. 3. C: 0.01 to 0.2% by weight, S
i: 0.2 to 0.6%, Mn: 0.3 to 2.0%, so
l. Al: 0.01 to 0.1%, and further, Cu:
One or two of 0.1 to 1.0% and Cr: 0.1 to 2.0%, and the remainder is inevitable impurities (however, 0.0% of S
After extruding a continuous cast slab or ingot made of Fe and hot forged or hot rolled (Ar
(3 points −30 ° C.) or higher temperature range (Ar 3 points −3 ° C.)
0 ° C) or more and 400 to 60 at a cooling rate of 5 to 25 ° C / s.
A method for producing a steel for a crude oil tanker having excellent fatigue crack propagation resistance in a wet hydrogen sulfide environment, wherein the steel is accelerated and cooled to a temperature range of 0 ° C. and then cooled or gradually cooled. 4. C: 0.01-0.2% by weight, S
i: 0.2 to 0.6%, Mn: 0.3 to 2.0%, so
l. Al: 0.01 to 0.1%, and further, Cu:
0.1-1.0% and Cr: 0.1-2.0. % Or Nb: 0.01 to 0.1%, Ti:
0.01 to 0.1% and V: 1 of 0.01 to 0.1%
Species or more, and the balance is inevitable impurities (however, 0.0 of S
After extruding a continuous cast slab or ingot made of Fe and hot forged or hot rolled (Ar
(3 points −30 ° C.) or higher temperature range (Ar 3 points −3)
0 ° C) or more and 400-60 at a cooling rate of 5-25 ° C / s.
A method for producing a steel for a crude oil tanker having excellent fatigue crack propagation resistance in a wet hydrogen sulfide environment, wherein the steel is accelerated and cooled to a temperature range of 0 ° C. and then cooled or gradually cooled.