JPH07188837A - Steel plate for pressure vessels with good crack resistance under stress in wet hydrogen sulfide environment - Google Patents

Abstract

PURPOSE:To produce a thick steel plate for pressure vessel excellent in cracking resistance under stress in a wet hydrogen sulfide environment. CONSTITUTION:This steel plate has a composition consisting of, by weight, 0.08-0.25% C, 0.1-0.5% Si, 0.8-1.6% Mn, 0.1-0.35% Cu, 0.05-0.35% Ni, 0.02-0.4% Cr, 0.02-0.3% Mo, 0.005-0.05% Al, 0.0005-0.008% Ca, 0.0001-0.0010% B, <0.015% P, <0.005% S, 0.002-0.01% N, and the balance Fe with inevitable impurities or further containing one or two kinds selected from the strength improving element group consisting of 0.01-0.05% V and 0.005-0.05% Nb.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet used in a pressure vessel of a petroleum refining apparatus or the like in a wet hydrogen sulfide corrosive environment, and relates to a thick steel sheet excellent in hydrogen-induced cracking resistance under stress. is there.

[0002]

2. Description of the Related Art The quality of crude oil is decreasing year by year and the concentration of hydrogen sulfide is increasing. Therefore, the pressure vessel of the oil refinery is also required to have resistance to a wet hydrogen sulfide corrosive environment, that is, resistance to hydrogen-induced cracking (HIC resistance). As measures against this, suppression of hydrogen invasion by addition of Cu and Ni, spheroidization of inclusions by Ca and REM treatment (for example, JP-A-54-31020 and JP-A-54-38).
No. 214, etc.), relaxation of segregation in the microsegregation portion, N
It is said that the addition of b, as-rolled, and the refinement of the structure until normalization are effective. Further, as a method for producing an extra-thick steel sheet having excellent HIC resistance, Japanese Patent Application No. 3-99296 has already proposed a reduction method depending on the amount of hydrogen in the steel.

On the other hand, when stress such as residual stress acts on a steel material in a wet hydrogen sulfide corrosive environment, hydrogen induced cracking (SOHIC) occurs under the stress. The above HIC resistance improvement measures are also effective for SOHIC, but SA5
It is not sufficient to suppress SOHIC in steel composed of ferrite phase and pearlite phase such as 16-70 steel, and new S
OHIC-specific measures are required.

The present inventors have already filed Japanese Patent Application No. 4-91273.
Japanese Patent Application No. 4-912274 and Japanese Patent Application No. 4-912274 propose measures to improve SOHIC resistance.
Although the effect of improving the IC property is remarkable, there is a problem that the manufacturing load is large.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a thick steel plate for a pressure vessel of a petroleum refining apparatus, which can suppress SOHIC (hydrogen-induced cracking promoted by stress in a wet hydrogen sulfide corrosive environment). The purpose is to do.

[0006]

The inventors of the present invention investigated the relationship between the B addition amount and SOHIC resistance in thick steel plates for pressure vessels used by normalizing treatment, and found that the appropriate B
It was found that the addition significantly improves the SOHIC resistance. The present invention was made based on this finding, and the gist thereof is as follows.

(1) C: 0.08-0.
25%, Si: 0.1 to 0.5%, Mn: 0.8 to 1.
6%, Cu: 0.1 to 0.35%, Ni: 0.05 to
0.35%, Cr: 0.02-0.4%, Mo: 0.0
2 to 0.3%, Al: 0.005 to 0.05%, Ca:
0.0005 to 0.008%, B: 0.0001 to
0.0010%, P: less than 0.015%, S: 0.
Less than 005%, including N: 0.002-0.01%,
A thick steel plate for a pressure vessel having good crack resistance under stress in a wet hydrogen sulfide environment, which comprises the balance Fe and unavoidable impurities.

(2) In% by weight, further V: 0.01
Of 0.05 to 0.05%, Nb: 0.005 to 0.05%, and 1 or 2 of the strength improving element group, the cracking resistance under stress in a wet hydrogen sulfide environment according to the above (1). Good steel plate for pressure vessels.

[0009]

The present invention will be described in more detail below.
0.12% C-0.25% Si-1.2% Mn-0.0
08% P-0.003% S-0.2% Cu-0.2% N
i-0.05% Cr-0.08% Mo-0.025% A
In a steel containing 1-0.002% Ca-0.003% N as a basic component, the amount of B added was changed. After normalizing from 900 ° C., the SOHIC resistance was evaluated.

The test piece for the SOHIC resistance test is a round bar test piece cut out from a steel plate and having a parallel portion of 6 mmφ × 25 mm length. A stress of 26 kgf / mm ² corresponding to about 0.7 times the yield strength of the steel material was applied to this test piece, and the breaking time in the NACE solution was determined. In addition, NA
The CE solution is an aqueous solution of 5% sodium chloride and 0.5% acetic acid saturated with hydrogen sulfide at 1 atm, and is generally used for the purpose of evaluating the resistance of steel materials to a wet hydrogen sulfide corrosion resistant environment. It is what

FIG. 1 shows the SOHIC resistance test result with respect to the amount of B added. As shown in the figure, when the amount of B added is 0.0001 to 0.001%, the breaking time in the SOHIC resistance test is improved. The reason for this is as follows. That is, when the B addition amount is less than 0.0001%, a pearlite phase having no SOHIC resistance improving effect appears in the microstructure, but B is 0.0001 to 0.00.
By adding 1%, the cementite spacing of the pearlite phase becomes small, and it becomes bainite in some cases. These phases propagate dislocations in the surrounding ferrite phase and
Improves OHIC property. On the other hand, the amount of B added is 0.001
If it exceeds%, island martensite (so-called MA structure) is generated, and this becomes the starting point of SOHIC.

It is necessary to limit the amount of B added to the above range, but it is within a range that can be sufficiently achieved by the general process capability of present steelmaking. The reasons for limiting the other constituent elements will be described below. C is an element effective for increasing the strength of the steel sheet, and in the case of steel for pressure vessels, 0.08% or more is added. However, if the addition amount is too large, the weldability is impaired, so the upper limit of the addition amount is 0.25%.

Si is added in an amount of 0.1% or more for deoxidation, but if the addition amount is large, the toughness decreases, so the upper limit is 0.5.
%. Mn is added in an amount of 0.8% or more in order to enhance the strength of the steel material, but if it exceeds 1.6%, the anisotropy of toughness increases, so the content is made 0.8 to 1.6%. Cu is an element that has the effects of increasing the strength of the steel material, improving the corrosion resistance, and reducing the amount of hydrogen that enters from the wet hydrogen sulfide environment. Therefore, 0.1% or more is added. However, if added in a large amount, hot workability is impaired, so the upper limit of the added amount is 0.35%.

Ni is an element that improves the toughness of the steel material and suppresses hydrogen penetration into the steel material, and is added in an amount of 0.05% or more. However, if it exceeds 0.35%, the effect tends to be saturated, so the upper limit is made 0.35%. Cr has the effect of increasing the strength. Therefore, 0.02% or more is added. However, if added in excess of 0.4%, the toughness decreases, so the upper limit is made 0.4%.

Like Cr, Mo is an element that increases the strength by addition, and is added in an amount of 0.02% or more. But,
If the content exceeds 0.3%, the cost increases, so the upper limit is set to 0.
3%. Al is an essential element for deoxidation of steel, and 0.005% or more is added for this purpose. However, 0.
The effect is saturated with the addition of more than 05%, so the range of addition is set to 0.005 to 0.05%.

P is less than 0.015% because it is an element that not only causes microsegregation in the steel to cause a significant difference in the toughness direction but also reduces the toughness. S forms less non-metallic inclusion MnS in steel, lowers the HIC resistance, increases the directionality of toughness, and lowers the upper shelf energy in the Charpy test, so S is made less than 0.005%.

N has the effect of forming Al and AlN and preventing the coarsening of crystal grains during normalization, and is added in an amount of 0.002% or more. However, if the addition amount is too large, the toughness may be lowered, so the addition amount is made 0.01% or less. Ca
Controls the shape of sulfide-based inclusions and is resistant to HIC and S
It has the effect of improving the OHIC property. 0.000
If the addition amount is less than 5%, no effect is observed, and if it exceeds 0.008%, the HIC resistance and the SOHIC resistance are adversely affected, so the addition range is set to 0.0005 to 0.008%.

Although the above-mentioned elements are used as basic components, one or two kinds of V and Nb having a strength improving effect may be added. V has the effect of forming carbonitrides and improving the strength of the steel material. When such an effect is required, 0.0
Add 1% or more. However, if it exceeds 0.05%, the toughness is adversely affected, so the upper limit is made 0.05%.

Like N, Nb forms carbonitrides and improves the strength of steel. Therefore, 0.005% or more is added, but if it exceeds 0.05%, the effect is saturated, so the addition amount is suppressed to 0.05% or less. Next, the manufacturing conditions of the material will be described. Steel having the above-mentioned chemical components is obtained by melting in a converter or an electric furnace, and then subjecting it to ladle refining or vacuum degassing treatment if necessary, and is continuously cast into a slab. Casting may be performed by a conventional mold or a unidirectionally solidifying mold, and in this case, a slab is formed by slabbing. A continuous casting slab may be agglomerated as necessary. Any soaking in the lumps may be used. That is, the steel ingot may be cooled and then subjected to soaking, or may be charged into the soaking furnace as a hot ingot. The soaking temperature is preferably 1000 to 1320 ° C.

Any reduction amount may be used in rolling, and the SOHIC resistance improving effect of the present invention is not impaired. After rolling, it is heated to a temperature of 850 ° C. or higher, and is left to cool to normalize it. Normalize the temperature to 850
The reason why the temperature is higher than or equal to ℃ is to make the structure uniform, and it is preferably 950 ° C. or lower from the viewpoint of finer structure.

[0021]

EXAMPLES Next, examples will be shown. Steels each having the chemical composition shown in Table 1 were melted in two batches, rolled into thick plates, and then subjected to normalizing treatment at the plate thicknesses and temperatures shown in Table 2. A round type SOHIC resistance test piece having a parallel portion of 6 mmφ × 25 mm length, cut out from these steel materials, has a weight of 26 kgf / mm.
A stress of ² was applied, the sample was dipped in the NACE solution described above, and the time until breakage was measured. The test continued until 720 hours.

The breaking time in the above SOHIC resistance test is shown in Table 2. Steel plates 1B and 3B have a B addition amount of 1
Less than ppm, SOH resistance due to the formation of pearlite phase
The IC property test resulted in a short break. Steel 2B and 4B
However, the B addition amount is large, and the breaking time is short at less than 100 hours due to the formation of island martensite, and the SOHIC resistance is poor.

On the other hand, in the steel sheets of 1A to 7A, the amount of B added is within the range of the present invention, it does not break at 720 hours, and the SOHIC resistance is excellent.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

INDUSTRIAL APPLICABILITY The steel sheet according to the present invention has good SOHIC resistance and is most suitable as a thick steel sheet for pressure vessels for petroleum refining equipment used in a wet hydrogen sulfide atmosphere. It has high safety and industrial value.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the amount of B added and the breaking time in a SOHIC resistance test.

Claims (2)

[Claims] 1. By weight%, C: 0.08 to 0.25%, Si: 0.1 to 0.5%, Mn: 0.8 to 1.6%, Cu: 0.1 to 0. .35%, Ni: 0.05 to 0.35%, Cr: 0.02 to 0.4%, Mo: 0.02 to 0.3%, Al: 0.005 to 0.05%, Ca: 0.0005 to 0.008%, B: 0.0001 to 0.0010%, P: less than 0.015%, S: less than 0.005%, N: 0.002 to 0.01%, and the balance A thick steel plate for a pressure vessel having good crack resistance under stress in a wet hydrogen sulfide environment, which is composed of Fe and unavoidable impurities. 2. The composition according to claim 1, further comprising, in wt%, one or two of a strength improving element group consisting of V: 0.01 to 0.05% and Nb: 0.005 to 0.05%. Steel plate for pressure vessel with good crack resistance under stress in wet hydrogen sulfide environment.