JPS6020459B2 - Sulfide stress corrosion cracking resistant high tensile strength steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The invention of this application relates to a high tensile strength steel that is resistant to stress corrosion cracking in an acidic liquid environment containing sulfides.

Stress corrosion cracking caused by hydrogen sulfide (QS) has become a problem in city gas spherical holders and LPG storage tanks made by welding heat-treated high-strength steel. In other words, for example, in the case of 80K9 class high tensile strength steel (HT80), cracks detected during an open inspection of a city gas spherical holder may occur at the toe of a multi-layer welded joint or tack weld (or jig installation trace). It has been believed that this is a hardened castle (Vickers hardness of about 40 Hv) which is a heat affected zone. This is HT60
None, HT8 Seio's heat-treated high-strength steels are heated from the austenitizing temperature to the welding preheating temperature of 100~
This is probably based on the fact that it is known that in the case of a hardened structure tempered at 150:00, the strength against stress corrosion cracking is significantly reduced, especially in an acidic aqueous solution containing sulfides. However, according to the results of the investigation conducted by the present inventor, the cracks observed in the above-mentioned city gas spherical holder are located at approximately Hv
It was found that at a softening temperature of 300 to 360, the structure was mainly a tempered martensitic structure.

According to a laboratory heat treatment reproduction test, this hardness corresponds to the hardness of a tempered martensitic structure of around 400 qo. However, it was confirmed that the strength of this structure against stress corrosion cracking was significantly reduced in acidic aqueous solutions containing sulfides. Based on the above findings, the present invention aims to provide a tempered high-strength steel that is less susceptible to stress corrosion cracking during tempering, in other words, is less prone to cracking even in acidic environments containing sulfides. 16% or less, Mno. 60-1.
20%, Cro. 60-1.20%, Moo. 30～
0.60%, Cu o. 15-0.50%, VO. 1
0% or less, BO. 006% or less, and Sio. 05%
Below, 0.01 to 0.1% of rare elements, the balance being substantially F
The present invention relates to a C-Mo-V tempered high-strength steel having a chemical composition of e and having high resistance to stress corrosion cracking against acidic liquids containing sulfides. The composition of the heat-treated high-strength steel according to the present invention is Si
Except for the difference in content and the fact that it contains rare earth elements, it is the same as the usual so-called Cr-Mo-V type 80 kg class heat-treated high tensile strength steel. That is, C is necessary to form fine carbides with base Fe in the scale martensite to form the required tempered structure, and its content should be 0.16% or less, preferably 0.10 to 0. It will be 14%.

If C is less than 0.1%, there is a risk that the strength will be insufficient, although there is a relationship with the content of other strength components;
If it exceeds this level, weldability will be impaired. Mn dissolves in the base, strengthens the base, and has the effect of removing sulfur and desulfurizing, but it is generally known that it is better not to have too much Mn in terms of resistance to sulfide stress corrosion cracking. If Mn is less than 0.6%, the strength will be insufficient, and if it is more than 1.20%, the strength will be excessive and the weldability will be poor, so the content should be 0.60 to 1.20%.

Cr is added to impart damping properties and competitive Z-back softening resistance through the formation of carbides, but if Cr is less than 0.6%, the strength will be insufficient, and if it is more than 1.20%, it will impair weldability. Therefore, it is set at 0.60 to 1.20%. M
o is also added for the same purpose as Cr, but Mo is 0.302
If it is less than 0.6%, the effect of imparting competitive softening resistance will be small and the strength will be insufficient, and if it is more than 0.6%, weldability will be impaired.

It is added for the purpose of strengthening the Cu'ma matrix and imparting weather resistance, but if it is less than 0.15%, there is no appreciable effect, and if it is more than 0.5%, it will particularly damage the rut resistance. Therefore, it is set to 0.15 to 0.50%. V is also necessary to produce carbides and show resistance to aging softening, and to maintain strength, and it is possible to improve weldability by reducing C, so 3 is added, and this is 0.10% or more. When the content becomes 0.10% or less, it particularly impairs the lubrication properties. B
is added for the purpose of imparting hardenability, but 0.006%
If it exceeds this, there is a risk that the strength will be exceeded, and at the same time, weldability will be significantly impaired, so the content should be 0.006% or less and 3 or less.

Also P. S is contained as an impurity in an amount of 0.030% or less as usual. The upper and lower limits of the content of these elements are limited to appropriate amounts that achieve the above-mentioned respective desired effects. , NK-HITEN
8M etc.). The high tensile strength steel of the present invention differs from conventional high tensile strength steels in that the Si content is kept low at 0.05% or less.

It has been known that reducing the Si content is effective in improving weld cold cracking susceptibility.

Therefore, in the steel of the present invention, Si is added in a targeted amount, or is not particularly added, and is incidentally contained at 0.05% or less. On the other hand, since Si acts as a deoxidizing agent during steel production, it was decided to add some rare elements that have deoxidizing and desulfurizing effects instead.

Rare earth elements (represented by RE) also have the effect of promoting the precipitation of fine carbides in the matrix, thereby reducing the amount of free C in the matrix and, as a result, reducing the precipitation of carbides at austenite grain boundaries. It is said. Since stress corrosion cracking in acidic solutions containing sulfides is mainly considered to be hydrogen embrittlement cracking that causes intergranular cracking, the present invention employs two methods: reducing the Si content and adding rare elements. We aimed for a synergistic effect by combining them.

If the rare element content is less than 0.01%, the effect of its addition is too small to be noticeable, and even if it is added more than 0.1%, no significant increase in the effect will be observed. The amount was 0.01-0.1%. Next, examples and test results will be explained.

Table 1 (wt omitted) Note.

Sample 1: Test material according to the present invention REO. 062PO. 0
07, SO. 004 Sample 2: Comparative material 1, no RE added, PO. 007, SO. 004 Sample 3: Comparative material 2, commercial product PO. 012, SO. Table 1 shows the chemical composition of the test material (sample 1) according to the present invention in comparison with that of the control material.

Sample 2 is a comparative material 1 having almost the same chemical composition as the test material according to the present invention, but no misch metal was added, and sample 3 is a Cr-Mo-V-based heat-treated high-temperature material. This is contrast material 2 of a commercially available tensile steel HT 8-fold product. In order to facilitate comparison, the test material and contrast material 1 (sample 2) had the basic composition of the commercially available contrast material 2 (sample 3), reduced Si, and were melted in a 50k9 high-frequency induction electric furnace.

Note that commercially available misch metal was used to make the test material contain rare elements. The molten steel was cast into 50k9 ingots with stamping marks, and then rolled into 17mm plates using a small rolling mill.

Each sample was heated at 1100 am for 30 minutes, cooled with water, and then tempered to each temperature. A notched round test piece (stress concentration factor Kt = 6.0) was fabricated from each sample plate heat-treated in this way, and Na2S spot 20 was added to dilute sulfuric acid as an accelerated test liquid (0.1% daily). ○40IQ ground: S) pH2,
A static constant load stress corrosion cracking test was conducted using a 20qo liquid, and the lower limit stress (OLcs) was determined and compared.

Table 2 shows the test results, and FIG. 1 plots the relationship between tempering temperature and lower limit stress.

As can be seen from Table 2 and Figure 1, the test material according to the present invention was tempered at 400mm
K9〆′Hiro's high.

Lcs, whereas the conventional comparative material 2 shows O.
Lcs was only 10kg/fence. In addition, the comparative material 1, which did not contain rare elements, had a resistance of 25[9〆/m], about 2.5 times that of the commercial product. death. s, but it was not as high as 35k9/n of the test material containing rare earth elements, and a difference in effect depending on whether or not rare earth elements were added was clearly recognized. Regarding the hardness of sample 1 according to the present invention, it is 40
Hv345 for phosphorus regeneration at 0°C x lhr, heating and air cooling
As mentioned above, although the hardness of the crack occurrence point in the open inspection of the city gas spherical holder is approximately equivalent to Hv 300 to 360, as mentioned above, the lower limit stress is higher than that of the conventional product. It can be seen that the value is shown.

From the above results, the high-strength steel of the present invention has the same effect as when heat-treated high-tensile steel is welded, heated to about 400°C and then air cooled, that is, heated back to 40,000°C. In stress corrosion cracking tests, it shows a significantly higher lower limit stress than its predecessor, and has strong resistance to stress corrosion cracking caused by acidic aqueous solutions containing sulfides.

Therefore, when used in welded structures such as city gas spherical holders and LPG storage tanks, great effects can be obtained in terms of resistance to sulfide stress corrosion cracking. Note that the Sj content of the present invention is 0.
0.05% or less and rare elements to 0.01
Adding ~0.1% improves the stress corrosion cracking sensitivity caused by medium temperature tempering, which is observed not only in Cr-Mo-V certified high-strength steels but also in low-alloy tempered high-strength steels. It is easy to understand that all of the things that can be done are thermal refining types.

[Brief explanation of drawings]

FIG. 1 is a graph showing the stress corrosion cracking test results. Figure/Figure

Claims (1)

[Claims] 1 C0.10-0.16%, Mn0.60-1.20%, Cr0.60-1.20%, Mo0.30-0.60%, Cu0.15-0.50 %, V0.10% or less, B0.006% or less, and Si0
．． 05% or less, rare earth elements 0.01 to 0.1%, and the remainder substantially Fe (weight%), Cr-, which has high stress corrosion cracking resistance against acidic liquids containing sulfides.
Mo-V type tempered high tensile strength steel.