JPS6013419B2 - Structural steel with improved cracking resistance in the Z direction - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a structural steel with improved cracking resistance in the Z direction, such as lamellar tear resistance or hydrogen-induced cracking resistance.

At the node portion of an offshore structure, a load acts in the Z direction (meaning the plate direction, hereinafter referred to as the Z direction), and lamellar tear is likely to occur.

Furthermore, in a sour gas environment including 2S, hydrogen-induced cracking parallel to the plate thickness is likely to occur. In order to eliminate these cracks, the S% was lowered to about 0.006 to 0.010% and REMO. 005-0.10% or Cao. 00
Adding 0.07% to 0.005% was a known technique. However, with conventionally known technology,
In order to add a large amount of REM and Ca, A such as MnS
On the other hand, the number of B-based inclusions decreases, but on the other hand, large B-based inclusions increase, and these inclusions gather locally, especially at the bottom of the steel ingot and near the surface of the lotus coin, causing ultrasonic defects. , rather promotes lamellar tear and hydrogen induced cracking, and these Z
It is often not a solution to directional cracks.

In particular, in the case of hydrogen cracking, research has shown that hydrogen cracking in line pipes under low pH acidic environments, which has become a particular problem in recent years and an industrial solution is urgently needed, occurs only from A-based or B-based inclusions. Since this has been clarified, in order to solve this problem, a technology has been required to change not only A-based inclusions but also B-based inclusions to C-based inclusions. The present invention is an attempt to fundamentally improve these drawbacks of the prior art, and as shown in Table 1, basically, the present invention first reduces S to 0.0020% or less, which was previously unthinkable. In addition, the amount of oxygen has been lowered to an unprecedented level to be less than 0.0035%, and this is combined with an extremely small amount of REM (0.0003 to 0.002%). By adding 0.0010 to 0.0070% of Ca, it is intended to provide a steel material with significantly better cracking resistance in the Z direction than in the past.

Table 1 Next, the reason for limiting the component composition of the present invention will be explained.

C is included as a component that can provide the strength necessary for structural steel at low cost, but CO. If it is less than 0.02%, the strength will be insufficient, while CO. If it exceeds 20%, the quality and weldability deteriorate, so C% was limited to 0.02 to 0.20%.

Si and Mn are important as deoxidizing elements and are also necessary components to ensure strength and quality. Sio. 01% or more is required, while Sil.
If the Sil. It was limited to 0% or less.

Mn is an important element for fixing S as MnS, but if Mn% is less than 0.3% or more than 2.0%, it will cause property deterioration such as insufficient rutting, so Nh% should be reduced to 0.3% or more than 2.0%. 3
It was limited to ~2.0%. In the present invention, the S content is 0.
0020% or less, but in order to reduce as much as possible the sulfides of A-based inclusions, which are one of the starting points of lamellar tear and hydrogen-induced cracking, the S% was particularly limited to an extremely low S content. .

(Particularly desirable is SO.0015% or less.) The reason why the P content is limited to a low content range of 0.020% or less is as follows. Lamellar tear and hydrogen-induced cracking
It was discovered that once generated from a sulfide, it propagates through a structure with a high P content during the propagation process, and furthermore, such a segregated part of P can reduce the P content to 0. .02
It was found that if the content was limited to 0% or less, the content decreased significantly, so the P% in steel was set to 0.020% or less. Regarding oxygen,
The occurrence points of lamellar tear and hydrogen-induced cracking may be caused by oxide inclusions, and therefore, it is effective to reduce the oxygen content to 0.0035% or less in order to eliminate the occurrence of these cracks.

Another reason is that when adding REM and Ca, by lowering the oxygen in the molten steel by vacuum degassing etc.
To prevent the formation of inclusions containing REM and Ca oxides, thereby eliminating B-based inclusions that tend to occur at the bottom of steel ingots and the surface layer of lotus coins, and to make the effects of REM and Ca particularly effective. Therefore, the oxygen content is particularly low and limited to 0.0035% or less. Alsolo. 15%
The following elements are included for the purpose of deoxidizing and ensuring the required toughness. Next, Cao. 0010~0.0070%
, REMO. The combined addition of 0003-0.002% is
This is an important point for the present invention.

0.0010-0.0070%, which is considered to be a relatively large amount
The purpose of adding Ca along with REM is to improve lamellar tear resistance and hydrogen-induced cracking resistance.
ao. If the Ca content is less than 0.0010%, the effect of improving such properties is insufficient, and if the Ca content exceeds 0.0070%, it will cause an increase in cluster-like inclusions. Since crackability deteriorates, Ca
% was limited to 0.0010 to 0.0070%. Particularly preferably, Ca% is 0.0020 to 0.0060%. Next, REMO. 0003 to 0.002% of the above C
The reason for adding Ca in combination with a is to eliminate cluster-like B-based inclusions that tend to occur when a relatively large amount of Ca is added alone as described above. As a result, a structural steel can be obtained in which the lamellar tear resistance and hydrogen induced cracking resistance are significantly improved in all parts including the steel core and the lotus coin piece. If REM is less than 0.0003%, the lamellar tear resistance and hydrogen-induced cracking resistance may deteriorate at the bottom of the steel ingot or the surface of the lotus coin when a large amount of Ca is added as described above. But REM is 0.000
By adding 3 to 0.002%, such deterioration is eliminated and significantly superior properties can be obtained. However, RE
If the M content exceeds 0.002%, the above-mentioned combined effect of Ca and REM will disappear, and the lamellar tear resistance and hydrogen-induced cracking resistance will deteriorate again at the bottom of the steel ingot and the surface layer of the lotus coin. There is a tendency to This is the reason why the upper limit of the REM content is limited. C in the above range
The combined addition of a and REM improves the sour environment (pH
5.2) Boats 3.5 to 4 are in a more severe environment than 2).
Even in this case, steel without hydrogen-induced cracking can be obtained regardless of the orientation within the steel ingot.

In order to more clearly show the appropriate content ranges of Ca and REM, experimental data are shown in FIG. In the experiment shown in FIG. 1, NO. For steel with each component of 1 to 15,
A steel plate containing the contents of Ca and REM as shown in the figure and rolled from a bottom piece of steel soul 2 times was used as a sample.
A time immersion test was conducted. It is clear from FIG. 1 that hydrogen-induced cracking does not occur within the scope of the present invention. No.
Table 2 (Chemical components wt %) In claim 2, in addition to the components in claim 1, Tio. 15% or less, Nbo. 1%
Below, VO. 1% or less, Cuo. 7% or less, Nio. 3
% or less, Crlo% or less, Mol. 0% or less, BO. 0
It is necessary to contain one or more elements selected from the group 0.05% or less.

All of these elements are added to structural steel to ensure strength, toughness, and weldability, as well as to further improve hydrogen cracking resistance in the Z direction. The reasons for adding each element and the reasons for restricting the components will be described below.

First, the purpose of adding Ti is to improve lamellar tear resistance, hydrogen cracking resistance, strength, plowability, and weldability by making the steel grain finer.

Addition of more than 0.15% causes coarse precipitation of TIC and reduces hydrogen-induced cracking resistance, so the upper limit should be set at 0.
．． It was set at 15%.

Similarly to Ti, the purpose of adding Nb is to improve lamellar tear resistance, hydrogen cracking resistance, strength, toughness, and weldability by making the steel grain finer.

Addition of more than 0.1% will cause coarse NK precipitation and reduce hydrogen-induced cracking resistance, so the upper limit should be set at 0.1%.
And so.

Similarly to Ti and Nb, V is an element added to improve lamellar tear resistance, hydrogen cracking resistance, strength, class I properties, and weldability by granulating steel.

Addition of more than 0.1% will cause coarse precipitation of V4C3, which will actually reduce hydrogen-induced cracking resistance, so the upper limit should be set at 0.
．． It was set at 1%.

Further, Cu' is added to improve lamellar tear resistance and hydrogen cracking resistance. The mechanism is that it penetrates into the steel and reduces the amount of diffusible hydrogen, which lowers lamellar tear resistance and hydrogen cracking resistance, by forming a corrosion film. It is known that the film that is formed is one in which Cu is concentrated. The reason why the upper limit is set to 0.7% is that if it exceeds 0.7%, red heat embrittlement will occur in the normal manufacturing process.
Further, Ni is added to increase lamellar tear resistance and hydrogen cracking resistance through the same mechanism as Cu, and also to increase strength and whipping properties.

The higher the amount of Ni added, the more the lamellar tear resistance can be improved, but considering the cost as a structural steel, it is appropriate to add 3% or less.Also, for Cr, the lamellar tear resistance and hydrogen cracking resistance Resistance and Strength 4 Toughness N
It is added to improve by the same mechanism as i.

The larger the amount of Cr added, the more the lamellar tear resistance can be improved, but as a structural steel, it is appropriate to add 10% or less in terms of cost. Mo is added to improve lamellar tear resistance and hydrogen cracking resistance by making the structure uniform.

That is, with the addition of Mo, the precipitation of ferrite from austenite during transformation progresses from the grain boundary to the grain in a needle-like manner, resulting in smaller microstructure units, and untransformed austenite is divided into small pieces, so that no band structure is formed and the microstructure becomes smaller. Uniformity can be achieved. However, when more than 1.0% of Mo is added, the above-mentioned ferrite-transformed portion becomes martensite, which easily causes cracks due to penetrating hydrogen. Therefore, the upper limit of Mo was set at 1%. Finally, B aims to improve lamellar tear resistance and hydrogen cracking resistance, as well as strength and rutting resistance, by making the structure granular. That is, intragranular precipitation of BN during the cooling process prevents the precipitation and growth of ferrite from austenite during transformation, and is beneficial for grain refinement. When the amount of B added exceeds 0.005%, intragranular precipitation of BN does not occur, but austenite grain boundary precipitation occurs, and the effect of inhibiting the growth of ferrite grains disappears.Therefore, the upper limit of B was set at 0.005%. .
The reasons for addition and limitations for each element have been described above, but the point is that Ti~Nb"V"Cu, NLCr, M
Each of the elements o and B is an element added for the purpose of improving lamellar tear resistance, hydrogen cracking resistance, and strength/grade properties through mechanisms such as string-stripe granulation, control of hydrogen penetration, and uniformity of structure. be. The steel of the present invention can be made into a coin ingot by conventional steel manufacturing, ordinary ingot making, continuous casting, or the like, and then subjected to a conventional rolling process to obtain a steel material with desired dimensions and shape.
Examples are shown below to explain the effects of the present invention in more detail.

Example 1 For steel having the components shown in Table 3, a hydrogen-induced cracking test was conducted on a steel plate taken from the bottom of the steel ingot under the environment shown in Table 5, and the results shown in Table 4 were obtained.

Table 3 (Chemical composition wt) Table 4 Note: The above numbers are the total crack length (average value of 3 cross sections: material/2
1.5×20).

Table 5 Example 2 A tensile test in the Z direction (thickness direction) was conducted on the steel plate (thickness 32 side) having the composition shown in Table 6 as a lamellar tear resistance test, and the aperture value (RAZ value) in the Z direction was determined. The results are shown in Table 7.

Excellent results have been obtained with the steel of the invention. Table 6 (Chemical components wt%) Table 7

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between hydrogen-induced cracking and the contents of Ca and REM. Figure 1

Claims (1)

[Claims] 1 C0.02-0.20%, Si0.01-1.0%
, Mn0.3-2.0%, S0.0020% or less, P0
．． 020% or less, oxygen 0.0035% or less, Alsol
0.15% or less, Ca0.0010-0.0070%,
Structural steel with improved cracking resistance in the Z direction, containing 0.0003 to 0.002% REM, with the remainder consisting of iron and unavoidable impurities. 2 C0.02-0.20%, Si0.01-1.0%
, Mn0.3-2.0%, S0.0020% or less, P0
．． 020% or less, oxygen 0.0035% or less, Alsol
0.15% or less, Ca0.0010-0.0070%,
Contains REM 0.0003 to 0.002%, and in addition, Ti 0.15% or less, Nb 0.1% or less, V0.
1% or less, Cu 0.7% or less, Ni 3.0% or less, Cr
It contains one or more elements selected from the group of 10% or less, Mo 1.0% or less, B 0.005% or less, and the cracking resistance in the Z direction is improved, with the balance being iron and unavoidable impurities. Structural steel.