JPH04329826A - Production of extra thick steel plate for pressure vessel excellent in hydrogen induced cracking resistance - Google Patents

Abstract

PURPOSE:To obtain an extra thick steel plate for pressure vessel excellent in hydrogen induced cracking resistance by performing rolling at the total reduction ratio corresponding to hydrogen content in the steel. CONSTITUTION:An ingot of steel having a composition composed essentially of 0.08-0.25% C, 0.1-0.5% Si, 0.8-1.6% Mn, 0.05-0.3% Ni, 0.005-0.04% Nb, 0.1-0.3% Cu, 0.005-0.05% Al, 0.0005-0.01% Ca, <0.01% P, <0.005% S, 0.002-0.01% N, and <0.00013% H is rolled so that the total reduction ratio (steel ingot thickness/product thickness) takes a value higher than both 4 and 5X[hydrogen content (ppm) in the steel]+1.5. After rolling, the resulting plate is reheated to >=850 deg.C and normalized.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an extra-thick steel plate with excellent hydrogen-induced cracking resistance for use in pressure vessels for petroleum refining and the like in a humid hydrogen sulfide corrosive environment.

0002

BACKGROUND OF THE INVENTION The quality of crude oil is decreasing year by year, and the concentration of hydrogen sulfide is increasing. For this reason, pressure vessels of petroleum refineries are also required to have resistance to wet hydrogen sulfide corrosive environments, that is, resistance to hydrogen-induced cracking (HIC resistance).

The HIC resistance of steel materials has been well investigated in the field of steel for line pipes, including (1) suppressing hydrogen intrusion by adding Cu and Ni, (2) making inclusions spherical by Ca and REM treatment (for example, 31020, Japanese Patent Publication No. 54-38214
etc.), (2) relaxation of segregation in micro-segregation areas, (2) refinement of as-rolled and as-normalized structures by adding Nb, etc. have been found to be effective in improving HIC resistance. These are also effective for pressure vessel steels, such as Cu, Ni,
Nb and Ca are often added.

[0004] However, these findings concern steel materials with a thickness of about 20 mm at most. On the other hand, steel plates for pressure vessels are extremely thick materials with a thickness of several tens to hundreds of millimeters, and their manufacturing processes and microstructures are different. It is not possible to manufacture extra-thick steel plates for pressure vessels that have the same

[0005]

[Problems to be Solved by the Invention] The present invention provides a manufacturing method for improving hydrogen-induced cracking resistance (HIC resistance) in a wet hydrogen sulfide corrosive environment in extra-thick steel plates for pressure vessels of oil refinery equipment. It's about doing.

[0006]

[Means for Solving the Problem] As a result of a detailed investigation into the effects of chemical composition and rolling conditions on the HIC resistance of extra-thick steel plates for pressure vessels used through normalizing treatment, the present inventors found that It has been found that the combination of hydrogen content and total rolling reduction ratio has a significant effect on HIC resistance even in steel ingots.

The present invention was made based on this knowledge, and in terms of weight percentage, C: 0.08 to 0.25%, Si:
0.1-0.5%, Mn: 0.8-1.6%, Ni: 0
．． 05-0.3%, Nb: 0.005-0.04%, C
u: 0.1-0.3%, Al: 0.005-0.05%
, Ca: 0.0005-0.01%, P: less than 0.01%, S: less than 0.005%, N: 0.002-0.01
%, H: less than 0.00013%, remaining Fe and unavoidable impurities, the total reduction ratio (ingot thickness/product thickness) from blooming to finish rolling is 4 and 5 x hydrogen content in steel. (p
Production of an extra-thick steel plate for pressure vessels with excellent resistance to hydrogen-induced cracking, characterized in that it is rolled so that it is larger than both pm) + 1.5, and then reheated to 850°C or higher and normalized after rolling. Further Cr: 0.02-0.2% depending on method and necessity
, Mo: 0.02-0.2%, V: 0.01-0.1%
This is a method for producing an extra-thick steel plate for pressure vessels having excellent hydrogen-induced cracking resistance and containing one or more of the strength-improving element group consisting of:

[0008]

[Operation] The present invention will be explained in more detail below. Figure 1 shows 0.18%C-0.15%Si-1.15%M
n-0.01%P-0.002%S-0.25%Cu-
0.15%Ni-0.02%Nb-0.02%Al-0
．． In steel containing 0.004%N-0.003%Ca (product plate thickness 100mm, normalizing temperature 880℃),
The presence or absence of defects in ultrasonic flaw detection before and after the HIC resistance test is shown for the combination of hydrogen amount and total reduction ratio (steel ingot thickness/product thickness) in the steel ingot. × indicates a defect is discovered by ultrasonic waves even before the HIC resistance test, △ indicates a defect is not found before the HIC resistance test but defects are discovered by ultrasonic wave after the HIC resistance test, ○ indicates It shows that no defects were found before and after the HIC test.

[0009] The immersion test piece for the HIC resistance test was 10 mm thick x 20 mm wide x 100 mm cut from the center of the thickness of the steel plate.
mm length, and the rolling direction and the length direction of the test piece, and the width direction of the steel plate and the width direction of the test piece are made to match. This test piece was immersed in a NACE solution for 96 hours to evaluate HIC resistance. Note that the NACE solution is an aqueous solution of 5% common salt and 0.5% acetic acid saturated with hydrogen sulfide at 1 atm.
This is commonly used in testing. CLR was used here as a parameter indicating HIC sensitivity. CLR is a value determined by the following equation.

CLR (%)=(Σai/A)×100 However, Σai
: Total length of step cracks (mm) A: Width of test piece (mm) CLR is the three cross sections of the above test piece (the cross-sectional shape is 10
mm x 20 mm) and calculated as an average value. When this average value was less than 1%, it was determined that there was no cracking. HI resistance
The cracks before the C test were also judged using the above criteria.

As shown in FIG. 1, the amount of hydrogen in the slab before rolling has almost no effect on defects in the ultrasonic flaw detection test before the HIC resistance test, and there are no defects when the total rolling reduction ratio is 4 or more. That is, a total reduction ratio of 4 is required to compress the porosity accompanying solidification. However, after the HIC resistance test, the total reduction ratio that can suppress defects is influenced by the amount of hydrogen in the steel ingot. In other words, if the amount of hydrogen in the steel ingot is small, HIC can be achieved even with a small total reduction ratio.
does not occur.

On the other hand, when the amount of hydrogen in the steel ingot is large,
It is necessary to make the total reduction ratio sufficiently large. When the amount of hydrogen in the steel ingot is H (ppm), a total reduction ratio of 5×H+1.5 or more is the boundary for defect generation in the HIC resistance test. Therefore, in order to ensure that no defects are present in the ultrasonic flaw detection both before and after the HIC resistance test, it is necessary to roll the steel to a total reduction ratio greater than both 4 and 5×H+1.5.

The reason for this is considered as follows. That is, if the total reduction ratio is not sufficiently large, even if the porosity is compressed by blooming and rolling, an interface remains, and hydrogen segregates at this interface. The greater the amount of hydrogen in the steel ingot, the greater the hydrogen segregation, and even if a small amount of hydrogen enters in the HIC resistance test, the amount of hydrogen at this interface will reach a critical concentration, and cracks will easily occur along this interface. occurs in Therefore, when the amount of hydrogen in the steel ingot is large, it is important to make the total reduction ratio sufficiently large to eliminate the interface.

[0014] If the amount of hydrogen in the steel ingot is too large, it will cause breakage such as splitting the steel ingot, so the upper limit of hydrogen amount that will not cause such breakage of the steel ingot is 0.00013.
% (1.3 ppm).

The reasons for limiting the component elements will be described below. C is an effective element for increasing the strength at room temperature and high temperature, and in the case of steel for pressure vessels, it is preferably added in an amount of 0.08% or more. However, if the amount added is too large, weldability will be impaired, so the upper limit of the amount added is set at 0.25%.

[0016] Si is added in an amount of 0.1% or more for deoxidation, but if the amount added is too large, the toughness decreases, so the upper limit is set to 0.5%. Mn is an effective element for fixing S and increasing strength, but if added in a large amount, it will cause significant segregation within the material and increase the anisotropy of toughness. do.

[0017] Since P is an element that not only micro-segregates in steel and causes significant directional differences in toughness, but also reduces toughness, the upper limit is set to less than 0.01%. S forms non-metallic inclusions MnS in steel, reduces HIC resistance, increases the directional difference in toughness, and lowers the upper shelf energy in the Charpy test, so the upper limit is set to less than 0.005%. do.

[0018] Cu is an element that has the effect of increasing the strength of steel materials, improving corrosion resistance, and reducing the amount of hydrogen penetrating from a wet hydrogen sulfide environment. For this reason, 0.1% or more is added. However, if added in a large amount, hot workability will be impaired, so the upper limit of the amount added is set at 0.3%.

Ni is an element that improves the toughness of steel materials and suppresses hydrogen penetration into steel materials, and when such effects are required, it is added in an amount of 0.05% or more. However, if it exceeds 0.3%, the effect begins to tend to saturate, so the upper limit should be set at 0.3%.
shall be.

[0020] Nb forms stable carbonitrides, prevents movement of grain boundaries, prevents coarsening of recrystallized grains, increases yield strength, and improves toughness. For this reason, 0.005% or more is added, but if it exceeds 0.04%, the effect will be saturated, so
The amount added is suppressed to 0.04% or less.

Al is an essential element for deoxidizing steel, and for this purpose it is added in an amount of 0.005% or more. However, 0.
Since the effect is saturated when added in excess of 0.05%, the addition range is set to 0.005 to 0.05%.

[0022] N forms Al and AlN and has the effect of preventing coarsening of crystal grains during normalizing, and is added in an amount of 0.002% or more. However, if the amount added is too large, the toughness may be reduced, so the amount added is set to 0.01% or less.

[0023] Ca controls the shape of sulfide inclusions and improves HI resistance.
It has the effect of improving C properties. If the addition amount is less than 0.0005%, no effect will be observed, and if it exceeds 0.01%, it will actually impair HIC resistance, so the addition range should be reduced to 0.0005%.
~0.01%.

The above elements are the basic components, but one or more of Cr, Mo, and V, which have the effect of improving strength, may be added.

Cr has the effect of increasing strength. When such an effect is required, 0.02% or more is added. However, if more than 0.2% is added, the toughness decreases, so the upper limit is set to 0.2%.

Like Cr, Mo is an element whose strength increases when added, and is added in an amount of 0.02% or more as necessary. However, since adding more than 0.2% increases the cost, the upper limit is set at 0.2%.

V forms carbonitrides and has the effect of improving the strength of steel materials. If you need this kind of effect,
Add 0.01% or more. However, if it exceeds 0.1%, the toughness will be adversely affected, so the upper limit is set at 0.1%.

Next, the manufacturing conditions of the material will be described. Steel with the above chemical composition is obtained by melting in a converter or electric furnace, then subjecting it to ladle smelting or vacuum degassing treatment as necessary, and is usually formed into an ingot using a mold or one-way solidification mold. Afterwards, it is made into slabs by blooming. Any type of soaking may be used in the blooming process. That is, the steel ingot may be soaked after being cooled, or the hot ingot may be charged into a soaking furnace. Soaking temperature is 1
It is desirable to set it as 000-1320 degreeC.

The amount of reduction in rolling is set to be the total reduction ratio determined according to the amount of hydrogen in the steel ingot before rolling, as described above. After rolling, heating to a temperature of 850°C or higher,
A normalizing process is performed by cooling. The reason why the normalizing temperature is 850° C. or higher is to make the structure uniform, and it is preferably 950° C. or lower in order to make the structure finer.

[0030]

[Example 1] Steel having the chemical composition shown in Table 1 was rolled to the total reduction ratio shown in Table 2, and normalized at the temperature shown in the table.

[0031]

[Table 1]

[0032]

[Table 2]

The steel plates of the steel plate series 1A to 11A are the steels of the present invention, and no cracking was observed in the HIC resistance test. On the other hand, the B series steel plates have a total reduction ratio smaller than the minimum value required to ensure HIC resistance, and cracks occur in the HIC resistance test.

[0034]

Effects of the Invention The steel sheet according to the present invention has good HIC resistance and is optimal as an extra-thick steel sheet for pressure vessels used in oil refining and the like used in a humid hydrogen sulfide atmosphere. It is highly safe and has great industrial value.

[0035]

[Brief explanation of charts]

[0036]

FIG. 1 is a chart showing the presence or absence of defects in ultrasonic flaw detection tests before and after HIC resistance tests for combinations of hydrogen content in steel ingots and total rolling reduction ratio (steel ingot thickness/product thickness).

Claims (2)

[Claims] Claim 1: C: 0.08-0.08% by weight.
25%, Si: 0.1-0.5%, Mn: 0.8-1.
6%, Ni: 0.05~0.3%, Nb: 0.005~
0.04%, Cu: 0.1-0.3%, Al: 0.00
5-0.05%, Ca: 0.0005-0.01%, P
: less than 0.01%, S: less than 0.005%, N
:0.002-0.01%, H:0.0001
A steel ingot consisting of less than 3% Fe, residual Fe and unavoidable impurities is processed by a steel ingot with a total reduction ratio (ingot thickness/product thickness) from blooming to finish rolling of 4 and 5 x amount of hydrogen in steel (ppm) + 1.5. A method for producing an extra-thick steel plate for a pressure vessel having excellent resistance to hydrogen-induced cracking, characterized by rolling the steel plate so that it is larger than both sides, and normalizing it by reheating to 850° C. or higher after rolling. Claim 2: Cr: 0.02 to 0.2 in weight%
%, Mo: 0.02-0.2%, V: 0.01-0
．． 2. The method for producing an extra-thick steel plate for pressure vessels having excellent resistance to hydrogen-induced cracking according to claim 1, which contains one or more of the strength-improving element group consisting of 1%.