JPS6254066A - High strength steel excellent in sulfide stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To obtain the titled high-strength steel for welding construction relatively inexpensive and having high toughness by specifying a composition consisting of C, Si, Mn, P, S, Ni, Cr, Mo, Al, Ti and Fe. CONSTITUTION:The steel has a composition consisting of, by weight, 0.05-0.15% C, 0.10-1.00% Si, 0.10-1.50% Mn, <=0.010% P, <=0.005% S, 0.30-3.00% Ni, 1.60-3.00% Cr, 0.01-0.10% Mo, 0.001-0.10% Al and/or 0.01-0.10% Ti and the balance Fe with inevitable impurities and further containing, if necessary, one or more between 0.01-0.10% Nb and 0.01-0.10% V and/or 0.0005-0.0050% B. The above steel is a high-strength steel combining high toughness with excellent sulfide stress corrosion cracking resistance and is capable of securing with certainly a tensile strength, after proper hardening and tempering, of about 60-80kgf/mm<2>.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a high tensile strength steel with excellent sulfide stress corrosion cracking resistance, particularly high toughness, and a tensile strength of 60 to 80 k.
g Regarding 17mm2 class high tensile steel with excellent sulfide stress corrosion cracking resistance, L in a humid environment containing hydrogen sulfide.
It is widely used in the manufacturing field of tanks for storing PG and city gas, steel structures that come in contact with petroleum products, as well as oil country pipes and line pipes for oil and natural gas.

[Conventional technology]

Oil, natural gas, and L produced using them as raw materials
Steel structures such as OCTG, pipes, and in-pipes that come into contact with petroleum products such as PG, tanks for crude oil, LPG, etc., and chemical plants that handle petroleum products are exposed to hydrogen sulfide generated in crude oil and during the refining process. It is known that this often causes sulfide stress corrosion cracking.

Resistance to sulfide stress corrosion cracking is closely related to hardness from the perspective of mechanical properties, and the fact that high hardness is more susceptible to cracking is explained in "Tetsu to Hagane" Vol. 54 (1968) No. 5. has been reported. Therefore, JP-A-55-73848, JP-A-58
107476, conventional measures against sulfide stress corrosion cracking of welded structural steel are aimed at making the heat-affected zone difficult to harden even when welded, and using a component with a low C equivalent. The common method was to adopt the composition.

In addition, from the perspective of alloying elements, the sulfide stress corrosion cracking resistance is particularly influenced by the presence or absence of N1 in the steel, as reported in the above-mentioned "Iron and Steel", and Ni1. Since the sulfide stress corrosion cracking resistance is expected to be significantly deteriorated, component compositions in which Ni is not added or are reduced are generally employed.

As mentioned above, many studies and disclosures have been made in the past as measures to prevent sulfide stress corrosion cracking in steel materials, and based on these studies and actual experience, as a preventive measure, the hardness of steel materials has been increased to 240 on the Vicker scale Hv. It is said that it is effective to limit the C equivalent to ~250 or less, and therefore, welded structural steel materials used in wet hydrogen sulfide environments have been required to have a sufficiently low C equivalent, taking into account the hardening caused by welding and the unsoundness of the structure. As a result, the tensile strength was 60%.
It was unavoidable to limit the weight to 17 mm or less.

In addition, although Ni is a typical alloying element that improves toughness, even steels with a tensile strength of 60 kg 17 mm2 are often made with no or reduced N1 added for the above reasons. As a result, low toughness in the weld heat affected zone could not be avoided.

However, steel structures for LPG, city gas, etc. are becoming larger and larger, and high-strength steel is economically advantageous when installing these steel structures. The current situation is that there is an urgent need for the development of a steel with excellent sulfide stress corrosion cracking resistance.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the problems of the above-mentioned conventional techniques of sulfide stress corrosion cracking resistant steel, to have high toughness, and to meet recent demands for tensile strength of 60 to 80 kg/17 m1112 class. The present invention aims to provide a low-alloy high-strength steel for welded structures with excellent resistance to sulfide stress corrosion cracking.

[Means for solving problems]

In order to solve the above problems, the present inventors first conducted research to reexamine the mechanism of sulfide stress corrosion cracking. At the time when the sulfide stress corrosion cracking phenomenon was first recognized, there were two theories: the hydrogen embrittlement theory and the active path cracking (hereinafter referred to as APC) theory. After that, many studies made the hydrogen embrittlement theory a common theory, but as a result of further research, the present inventors came to the following fact. In other words, the mechanism by which cracks occur in steel materials in an environment containing hydrogen sulfide (hydrogen embrittlement and APC both act; APC occurs initially and acts to form a place where hydrogen accumulates, promoting hydrogen embrittlement. In addition, Ni in steel, which is generally said to deteriorate sulfide stress corrosion cracking resistance, only has the effect of making APC more likely to occur, and has nothing to do with hydrogen embrittlement. I was able to clarify this.

Therefore, conventionally, prevention of sulfide stress corrosion cracking has focused on preventing hydrogen embrittlement cracking, but based on the above findings of the present inventors, it has been concluded that it is also necessary to prevent APC. As a result of repeated research from this point of view, the present inventors found that among alloying elements, Cr is effective as an element that suppresses the generation of APC, and conversely, MO is an element other than the above-mentioned N1 that promotes the generation of APC. It has been found. However, the N
i, M.

It was discovered that the adverse effects of Cr can be suppressed by adding 16% or more of Cr, and after further research, the present invention was completed.

The objective of the present invention is to achieve high toughness and tensile strength of 6.
A high tensile strength steel with sulfide stress corrosion cracking resistance of 0 to 80 kg f/mm2 class can be effectively achieved by the following four inventions.

The gist of the first invention is as follows.

That is, in terms of weight ratio, C: 0.05-015% Si: 0.10-100% Mn: 0.10-1.50% P: 0.010% or less S: 0.005% or less Ni: 0. 30-300% Cr: 1. 6 0-3. 0 0% Mo: 00
1-010% and Aj': 0.001-0.10%, T summer: 0.0
It is a high-strength steel that is characterized by high toughness and excellent resistance to sulfide stress corrosion cracking.

The gist of the second invention is that Hokani having the basic composition of the first invention,
Sarani Nb: o, 01~0.10%, V: 0.01~
The steel contains one or two selected from among 0.010% and the remainder consists of Fe and unavoidable impurities.

The gist of the third invention is that in addition to the basic composition of the first invention, B
:0.0005 to 0.0050%, with the remainder being Fe and unavoidable impurities.

The gist of the fourth invention is that, in addition to the basic composition of the first invention,
Further ICNb70.01~0.10%, V:0.01~0
One or two selected from 10% and B:O
,0O05~0.0050% at the same time, and the remainder is Fe.
and unavoidable impurities.

First, the reason for limiting the basic composition of the steel of the present invention will be explained.

C: C has the effect of ensuring strength by improving the quenching and tempering properties of steel, and for this purpose it must be at least 0.005%
Requires. However, if the amount exceeds 015%, it will coexist with 160% or more of Cr described below, hardening the weld heat affected zone, and eventually deteriorating the sulfide stress corrosion cracking resistance. limited to the range of

S1: Si is added for deoxidation and to increase strength, but for this purpose at least 0.1% is required. However, if the amount exceeds 100%, the toughness will deteriorate, so the upper limit is set at 1,00%, and 0.10 to 100%
limited to the range of

Mn: Like Sl, Mn has a deoxidizing effect and also has the effect of increasing strength and toughness, but if it is less than 10%, this effect cannot be obtained, and if it exceeds 150%, it will cause deterioration of toughness. Therefore, the upper limit is set to 150%, and 010~
It was limited to a range of 150%.

P: P is an impurity that inevitably accompanies steel, and the less the better, but if the content exceeds 001096, it will segregate at grain boundaries and easily cause cracks during processing. Since cracks may occur, the upper limit was set at 0010%.

S: Like P, S is an impurity that inevitably accompanies steel, and the smaller the content, the better.
%, it is a harmful component that forms MnS-based inclusions and stretches during hot rolling, inducing the starting point of cracking.
The upper limit was set at 0005%.

NI: Ni1. It is an element that promotes the hardenability of steel and contributes to improving the toughness of the base metal and the weld heat affected zone, and for this purpose, at least 0.30% is required. However, if the amount exceeds 30%, the effect will be saturated, so the upper limit is set at 30%, and 0.
It was limited to a range of 30-30%.

Cr: C' has the effect of increasing the tempering resistance and also has the effect of suppressing the occurrence of <A'PC' as described above.

Particularly in the presence of Ni and Mo, Ni has the effect of suppressing the APC generation and promotion effect of these elements, and is one of the important elements in the present invention. At least 160% is required for this effect to be effective. However, if it exceeds 30%, the toughness of the steel deteriorates, so the upper limit was set at 30%, and the range was limited to 160 to 30%.

MO: MO has the effect of increasing the tempering resistance of steel, and therefore requires at least 0.01%. But 010%
If it exceeds this amount, the weld heat-affected zone will be hardened, and the sulfide stress corrosion cracking resistance will be deteriorated.

Al: Al has a deoxidizing effect, and also combines with N in steel to form nitrides, and in the case of B-containing steel, it has the effect of activating the effect of B, but it especially has the effect of making crystal grains finer. Since it has a great effect of increasing toughness, it is added together with T1, either alone or in coexistence, if necessary. However, its content is 0001
If it is less than 0.1%, the above effect cannot be obtained; on the other hand, if it is less than 0.10%
If the amount exceeds 0.1%, it will lead to an increase in inclusions, which will eventually lead to deterioration of toughness, so the upper limit is set at 010%, and
It was limited to a range of 0.010%.

T I: Like AI, T1 forms nitrides, contributes to grain refinement, and has the effect of increasing toughness, and in the case of B-containing steels, together with B, it has the effect of improving hardenability and tempering resistance. Therefore, either one of them is added alone or in coexistence with kl as necessary.

However, if the content is less than 0.01%, the above effects cannot be obtained, while if the content exceeds 0.010%, the toughness will deteriorate, so the upper limit is set at 0.010%, and the content ranges from 0.01 to 0.10%.
limited within the range of

The above composition is the basic composition of the steel of the present invention, but if necessary, one or two of Nb and V and B may be separately added in the following limited amounts, or one of Nb and V may be added separately. By simultaneously adding one species or two species and B, the effects of the present invention can be further improved. The reasons for limiting the amounts of these selectively added elements are as follows.

Nb, V: Both Nb and V have the effect of improving the tempering resistance of steel, so they are added singly or in combination as necessary, but if they are less than 0.01%, the above effects will not be achieved. I can't get it. However, since these effects become saturated if the content exceeds 0.010%, both are limited to a range of 0.01 to 0.10%.

B: B has the effect of improving the hardenability and tempering resistance of steel, so it is added as necessary. However, the above-mentioned effects do not appear at less than o, o o o s%, and o,
Since these effects will be saturated even if added in excess of 0.0005% to 0.0050%.

Next, a method for producing the steel of the present invention having the above-mentioned limited composition will be explained.

The method for manufacturing the steel according to the present invention having a limited composition according to the present invention may be carried out by a conventional method, and there is no need for particular limitation, but it is preferable to carry out the method as follows. That is, after melting steel with the above-mentioned composition and sufficiently degassing, it is ingotted and bloomed while suppressing gas absorption, or it is made into steel slabs by continuous casting, and then it is hot-rolled. It shall be a steel plate of specified dimensions.

Thereafter, a quenching treatment is performed in which the steel plate is heated to a temperature in a range that does not cause coarsening of austenite crystal grains having an Ac3 transformation point or higher, and then rapidly cooled. The structure of the steel sheet obtained by this quenching treatment is mainly a martensitic structure, which is extremely hard and brittle and extremely sensitive to sulfide stress corrosion cracking.
C. Tempering at an appropriate temperature below the transformation temperature imparts toughness and solves the above problem. Through this tempering treatment, the steel of the present invention has a tensile strength of 60 to 80 kg.
17 mm2 can be obtained. As shown in the examples below, conventional steel with a tensile strength of 60 to 80 kg f/mm is 60 kg f/mm, which is low C of our company and without the addition of Ni.
Prevention of sulfide stress corrosion cracking, which was considered difficult with steels other than M2 grade steel, is now possible with the high strength steel of 60 to 80 kg and f/2, which is a major feature of the steel of the present invention. It is.

〔Example〕

Invention steel specimens A, B, CSD, E, 'F, G, H and comparative steel specimens 1. having the components and C equivalents shown in Table 1. JXK, L, and M were melted under the same conditions, and after degassing treatment, they were bloomed to suppress gas absorption, and then hot rolled to give specimens A, B1, and J a thickness of 25++ nn, and specimen C. , D, E, F.

G, H, 1L, and M were steel plates with a thickness of 38 mm.

The C equivalent in Table 1 is expressed by the following formula.

Each of these test materials was quenched under the conditions shown in Table 2.
After tempering, the mechanical properties were investigated by tensile test and Charpy impact test.

Furthermore, for these test steel plates, the welding heat input 1
Covered arc welding was performed under the condition of 2 kJ/cm, and the maximum hardness of the heat affected zone of the welded joint was measured.

The welding conditions in this case are as follows. That is, the groove depth is 10 mm on the surface of the steel plate, and the bottom shape is 5 mm R.

U-shaped groove cutting with a bevel angle of 50 degrees, welding heat input of 12
Overlay welding was performed at J/cm. The maximum hardness of the weld heat affected zone is as shown in Table 2. In addition, the tensile strength of the welding rods used and test materials A, B, and I is 60 kg.
f/nm+2m+2 for gold steel, and for the others, those with a tensile strength of 80 kg f/+nn+'' for gold steel were used.

Next, a sulfide stress corrosion cracking test was performed by cutting one surface layer from the welded joint and measuring 5 inm x 18 nmw + x 115
A test piece of mm in diameter was taken, and stress was applied to the test piece using a fixed four-point bending method. At this time, welding metal and welding heat w
The test was conducted so that the 6th sound part was located at the center 40 mm of the test piece where the stress distribution was even and negative.

Test solution is sufficiently degassed pure water or 05% acetic acid + 5%
It was adjusted by blowing mixed gases with different H2S partial pressures into the NaCl aqueous solution. This mixed gas is 999% pure N
Balanced with 2 and assuming a total pressure of 1 atm, 10ppm to 300
Aqueous solutions of 0 ppm H2S with different io degrees were prepared. The immersion time of the stress-strengthened test piece was 1000 hours, during which time the mixed gas was continuously blown into the test tank, and the temperature of the test aqueous solution was maintained at (25±1)°C. These various concentrations of H2
Table 3 shows the results of the stress corrosion cracking test using the steel of the present invention and comparative steel specimens in an S aqueous solution.

The values in Table 3 are expressed as a percentage of the load stress relative to the yield strength of the base material at the time of crack occurrence. The cracks in the test piece were observed using an optical microscope with a magnification of 400 times. Therefore, each test material shows that no cracking occurred at stress ratios below the stress ratio shown in Table 3.

As is clear from Table 3, in a low-concentration environment with a H2Sa degree of 100 ppm or less, even the best specimens, except for comparison steel sample I, crack when the stress ratio exceeds 80%.
Each specimen of the steel of the present invention has a stress ratio of io.

Even if it is %, it will not break at all. Also 500 PpI1
In an environment with a high concentration of hydrogen sulfide of 1 or more, the cracking critical stress ratio of each comparative steel specimen decreases in all environments except for comparative steel specimen I, but the steel of the present invention has a It shows the cracking critical stress ratio.

As described above, each test material made of the steel of the present invention has excellent resistance to sulfide stress corrosion cracking, and as shown in Table 2, its tensile strength reaches 64 to 87 kg 17 mm2, and its toughness also increases. The Nyoro fracture surface transition temperature in the Charpy impact test was as low as -85°C to -110°C, indicating high strength and toughness. On the other hand, although the comparative steels are comparable in tensile strength and toughness to the steels of the present invention, their resistance to sulfide stress corrosion cracking is significantly inferior to that of the steels of the present invention except for sample I.

Regarding specimen I, the sulfide stress corrosion cracking resistance is comparable to that of the steel of the present invention, but as shown in Table 2, the toughness value expressed by the fracture surface transition temperature is lower than that of the steel of the present invention. Significantly inferior to the sample material.

〔Effect of the invention〕

The steel of the present invention has a limited composition, and Cr: 1.60 to 300% is added as an element that suppresses the generation of APC, and N1 and MO, which were conventionally thought to promote the generation of APC, are added.
We have discovered that the negative effects of carbon can be suppressed by adding 16% or more of C. We have achieved sulfide stress corrosion cracking resistance with a relatively low-cost low-alloy steel, and the tensile strength after quenching and tempering is 60~60. We were able to secure 80kg of 7mm2 and obtain high tensile strength steel with high toughness, and specifically obtained the following effects.

(a) In an environment with a low concentration of hydrogen sulfide of 100 ppm or less, no cracking occurs even if the applied stress is the same as the yield strength of the base material. Further, even at a high concentration of 500 ppm or more, no cracking occurs under a load stress of 50% of the yield strength of the base material.

As such, it has significantly better resistance to sulfide stress corrosion cracking than conventional steel.

(b) Tensile strength after quenching and tempering is definitely 60-8
0 kg f/mm2, and conventional steel of the same type is 60 kg 17 m due to hardening due to welding and unsound structure.
We were able to solve the problem that had to be limited to less than m''.

(c) Fracture surface transition temperature by Charpy impact test is -85
~-110°C and has high toughness.

(d) Based on the above characteristics, the steel of the present invention can be used for storage containers and other welded structures in environments with low concentrations of hydrogen sulfide, such as LPG and city gas, while ensuring sufficient safety while reducing wall thickness (°
It is possible to set up economically.

Claims (4)

[Claims] (1) Weight ratio: C: 0.05-0.15% Si: 0.
10-1.00% Mn: 0.10-1.50% P: 0.010% or less S: 0.005% or less Ni: 0.30-3.00% Cr: 1.60-3.00% Mo: 0.01-0.10% and Al: 0.001-0.10%, Ti: 0.01
A high tensile steel containing one or two selected from ~0.10%, the remainder consisting of Fe and unavoidable impurities, characterized by high toughness and excellent sulfide stress corrosion cracking resistance. (2) Weight ratio: C: 0.05-0.15% Si: 0.
10-1.00% Mn: 0.10-1.50% P: 0.010% or less S: 0.005% or less Ni: 0.30-3.00% Cr: 1.60-3.00% Mo: 0.01-0.10% and Al: 0.001-0.10%, Ti: 0.01
Contains one or two selected from ~0.10%, further Nb: 0.01~0.10%, V: 0.01~0
．． A high-strength steel containing one or two selected from among 10%, the remainder consisting of Fe and unavoidable impurities, and characterized by high toughness and excellent resistance to sulfide stress corrosion cracking. (3) Weight ratio: C: 0.05-0.15% Si: 0.
10-1.00% Mn: 0.10-1.50% P: 0.010% or less S: 0.005% or less Ni: 0.30-3.00% Cr: 1.60-3.00% Mo: 0.01-0.10% and Al: 0.001-0.10%, Ti: 0.01
Contains one or two selected from ~0.10%, further contains B: 0.0005~0.0050%, and the remainder is Fe and unavoidable impurities, and is characterized by high toughness and sulfuration resistance. High tensile strength steel with excellent stress corrosion cracking resistance. (4) C: 0.05-0.15% Si: 0.
10-1.00% Mn: 0.10-1.50% P: 0.010% or less S: 0.005% or less Ni: 0.30-3.00% Cr: 1.60-3.00% Mo: 0.01-0.10% and Al: 0.001-0.10%, Ti: 0.01
Contains one or two selected from ~0.10%, further Nb: 0.01~0.10%, V: 0.01~0
．． One or two selected from 10% and B:
A high-strength steel containing 0.0005 to 0.0050%, the remainder consisting of Fe and unavoidable impurities, and characterized by high toughness and excellent resistance to sulfide stress corrosion cracking.