CA1171697A - Steels which are useful in fabricating pressure vessels - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Chromium-molybdenum steels which have high strength, good hydrogen attack resistivities, good high-temperature ductilities and high creep rupture strength, and which, when formed into steel plates, welded and given a high-temperature strain relief annealing treatment will acquire high resistivity to creep damage, comprises. 0.11 to 0.17% of C, 0.50 to 1.20% of Si, 0.46 to 1.00% of Mn, 1.00 to 1.50% of Cr, 0.45 to 0.65% of Mo, 0.00015 to 0.0015% of B, 0.010 to 0.100% of Sol, A1 and inevitably entrained impurities, the balance to make up ???
of Fe. Preferably the inevitable nitrogen and phosphorus impurities are kept below 0.0060% of N and 0.010% of 1.

Description

~ ~7~6~'7 BACKGROUND OF ~HE INVEN~ION
FI~LD OF ~HE INVENTION
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The present invention relates to steels which are useful in fabricating pressure vessels, and more particularly it relates to improved chromium-molybdenum type steels which can be formed into thick plates that are weldable together to make chemical pressure vessels.

~HE PRIOR AR~

Chemical pressure vessels, such as pressure vessels used as oil-refining reactors, must be fabricated from materials which excel in high temperature strength, creep strength and hydrogen-attach resistivity.~ The most widely used materials ; have been 14Cr-0.5Mo type steels, whlch are standarized steels as discussed, for example, ln AS~M A387 G11.
15In recent years, however, pressure vessels have been growing larger and larger in size, thus requiring the use of steel plates having increasing thlcknesses (e.g. of 100 mm or more). Conventional 14Cr-0.5Mo type steels, which are generally produced using normalizing and tempering treatments, have not produced completely satisfactory steel plates having such great thicknesses. ~his lS due to the fact that~as the plate thick-nesses increase, the steel plate cooling speed is reduced and the ferrlte content will lncrease during the normalizlng treatment. ~he temperature of the stress relief annealing after welding or post weld heat treatment (hereinafter abbreviated as PWH~)`thus must be elevated and the treatment duration lengthened~
As a result, the strength of the~produced steel will be drastical-ly lowered~ Consequently, the produced steel plates will often times not have the required strengths for use in making pressure 3o Ivessel . ~t the s me time, howev~ , increasing the content of ' ~'71 ~7 alloying elements such as chromium and molybdenum will not solve the problem because the weldability of the steel plates will be reduced.
In addition, it has recently been discovered that the occurrence of creep damage in pressure vessels in welded heat affected ZOlle (hereinafter abbreviated as HAZ), accompanied by stress concentration, has become a serious problem.
Investigations on the creep damage have revealed that at normal operation temperatures of pressure vessels (400 - 550C), the hardened zones suffer from much degradation of grain-boundary strength, and that owing to the concentration of strain in the grain boundaries, cracks develop at the boundaries which grow and mature into rup-tures. The most effective way to prevent creep ruptur~s is to enhance the strength of the base metal and to soften the HAZ by applying a high-temperature PWHT (6S0 - 720C).
With respect to enhancing the strength of the base metal, this can be achieved by lowering the phosphorus content.
However, drastically decreasing the phosphorus content (e.g., to less than 0.010%) entails huge cost increases because a special treatment step in the steelmaking operation becomes necessary.
One proposal for solving the noted problems is disclosed in JA-OS 41962/1980 (laid open for public inspection on March 25, lg80). In this proposal, the - employed Cr-Mo steels have aluminum and boron or boron and titanium added thereto. However, this proposal is basically directed to two ~Cr-lMo type steels which suffer from severe embrittlement from tempering treatment, and it is also directed to preventing ~his embrittlement .

~7~1 ~Y17 by lowering the silicon and manganese contents. When this prior art proposal for improving steel proper~ies is applied to ~Cr-0.5Mo steel plates which have ~hicknesses exceeding, for example, 50 mm, the steel plates will have insufficient strengths and toughnesses because of the low manganese contents. Indeed, because the silicon contents are kept low, the chromium and molybdenum contents must be increased to achieve the necessary s~rengths. This detrimentally increases the cost of production, and the produced steels still lack the extra strength needed for withstanding the high temperature PWHT and do not show improved creep damage resistivity.
It is thus an object of the present invention to provide improved Cr-Mo type steels which can be formed into extremely thick plates for use in fabricating pressure vessels, which plates will have increased strengths (even after high temperature PWHT) and enhanced creep damage resistivity, yet will not have reduced weldability or increased cost.
SUMMARY OF THE INVENTION
We, the present inventors, have found that Cr-Mo type steels, and especially 1 ~r~0.5Mo steels, will display all of the required properties when 0.00015 to 0.0015% of boron and 0.01 to 0.100% of Sol. Al have been incorporated therein after addition of at least 0.46~ of Mn. We have found that the addition of this specific amount o Sol. A1 and the noted minute amount of boron to the Cr-Mo steel curbs the ferrite transformation during the normalizing treatment, gives rise to a bainite structure and enhances the steells strength. We have found that when the cooling speed is lowered, the excess amount of boron is precipi-tated in the austenite grain boundary which promotes the ferrite transformation and lowers the strength. Therefore, the very minute boron addition proves advantageous in enhancing hardenability during normal.izing.
Thus, according to one aspect of the invention there is provided a high strength steel which also displays high hydrogen attack resistivity, good high-temperature ductility and high creep rupture strength, said steel being useful in fabricating pressure vessels, said steel consisting essentially of 0.11 to 0.17% of C, 0.50 to 1.20~ of Si, 0.46 to 1.00% of Mn, 1.00 to 1.50% of Cr, 0.45 to 0.65% of Mo, C.00015 to 0.0015% of B, 0.010 to 0.100~ of Sol, Al, 0 to 0.006% of N as an impurity and 0 to 0.010% of P as an impurity, said percentages being by weight, and the balance of Fe.
According to another aspect of the invention there is provided a steel plate being useful in fabricating pressure vessels, said steel plate having essentially the composition of 0.11 to 0.17% of C, 0.50 to 1.20% of Si, 0.46 to 1.00% of Mn, 1.00 to 1.50% of Cr, 0.45 to 0.65% of Mo, 0.00015 to 0.0015~ of B, 0.010 to 0.100% of Sol. Al, 0 to 0.006% of N as an impurity and 0 to 0.010% of P as an impurity, said percentages being by weight, and the balance of Fe and being used after a post weld heat treatment at a temperature within the range of 650 to 720C.

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BRIEF DESCRIPTION OF TH~ DRAWINGS
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Figure 1 is a diagram showing a heat pattern of "step cooling" that is an accelerating process to learn a temper embrittlement for short time, and Figure 2 is a diagram showing the relationship of tempering parameter and mechanical property between the con~
ventional steel and the steel of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have discovered that a steel composition containing 0.11 to 0.17% of C~ 0.50 to 1.20% of Si9 0.46 to 1.00%
of Mn, 1.00 to 1.50% of Cr, 0.45 to 0.65% of Mo, 0.00015 to 0.0015% of B, 0.010 to 0.100% Of Sol. Al and the balance-(to make up 100%) of Fe (together with impurities inevitably entrained by the components of the compositlon) gives highly favorable results.
A further explanation of the reason~s behind the required ranges noted for the carbon, silicon, manganese, chromium, polybdenum, boron and Sol. aluminum are as follows.
Carbon is a primary element for achieving the necessary product strength. To fulfil this purpose, carbon must be added in an amount of not less than 0.11%. If the carbon content exceeds 0.17%~ however, the excess carbon results ln reduced toughness and weldability. Thus, the carbon content must be from 0.11 to 0.17%.
Silicon is necessary, if not solely9 for deoxidation.
With the steel of the present invention, an important purpose of the use of silicon is to help ~the base metal acquire strength both at room temperature and at e evated temperatures. For this , .

~l7les7 purpose, the silicon must be added in an amount of not less than On50k~ If the silicon content exceeds 1.20%~ however, the excess silicon causes serious degradation in product toughness. Thus, the silicon content must be from 0.50 to 1.20%. ~urther, since the toughness of the mother matexial will be reduced to a slight extent when the silicon content exceeds 0.70/0, the preferred silicon content is from 0.50 to 0.70%. ~his amount will be used in steels in which the product toughness will b-e of greatest importance.
In order to have &n enhanced hardenability during the normalizing treatment &nd in order to help very thick steel plates to acquire strength and toughness, m&nganese must be added in &n amount of no-t less th&n 0.46%. If the mang&nese content exceeds 1.00%, however, the excess mang&nese degrades the product weld-ability. ~hus, the m&ng&nese content must be from 0.46 to 1.00%.
Molybdenum is necessary for heightening the product hardenability during the normalizing treatment &nd for enh&ncing its strength both at room temperature &nd at elevated tempera-tures. For this purpose, molybdenum must be added in &n amount of not less th&n 0.45%. If the molybdenum conten-t exceeds 0.65%, however, the excess molybdenum adds to the product's strength but causes a loss in its toughness. ~hus, the molybdenum content must be from 0.45 to 0.65%.
Chromium serves to heighten the product's hardenability ~25 during the normalizing treatment and to enhance hydrogen attack resistivity. To achieve these results, the chromium must be added in &n amount of not less th&n 1.00%. If the chromium content exceeds 1.50%, however, the excess chromium acts to degrade weldability (also, 1. 50% is the upper limit specified by the AS~M standard). ~hus, the chromium content must be from .

117~ 697 .00 to ~.~0/o. ~urther, since hydrogen attach resistivity increases in direct proportion to an increase in the chromium content (possible dispersion of chromium content under normal production conditions being also be taken into account), the preferred chromium content is from 1.25 to 1. 50%.
Boron is capable of enhancing the product's hardenabili-ty during the normalizing treatment as described above. To help the steel of this invention acquire the micro-structure consisting essentially of baitnite, boron must be added in an amount of not less than 0.00015%. If the boron content exceeds 0.0015%~ however the excess boron heightens susceptibility to PWH~ cracking, induces precipitation of ferrite during the normalizing treatment and degrades strength. ~hus, the boron content is from 0.00015 to 0.0015%. Further, to ensure the effect of boron upon the hardenability and lessen possible dispersion of strength, boron is preferably added in an amount of not less than 0~000~/0. When the boron content exceeds 0O0009%~ the excess boron acts to impair the toughness of the heat affected zone. ~herefore, boron is preferably used within the range of from 0.0002 to 0.0009%.
~0 ~he Sol. Al is necessary for fixing nitrogen in the form of AlN and for ensuring thorough deoxidation of the steel.
To fulfil this purpose, the Sol. Al must be added in an amount of not less than 0.010%. If the So. Al content exceeds 0.100%, however, the excess Sol. Al acts to impair hot workability and causes sùrface cracks on the steel slabs. ~hus, the Sol. Al content must be from 0.010 to 0.100%. ~0 ensure the possitive effects of boron, the Sol. Al is preferably added in an amount of not less than 0.035%. If the~Sol. Al content exceeds 0.065%, however, the excess Sol. Al acts to degrade the toughness of HAZ.
3Q ~hus, the preferred Sol. Al content is from 0~035 to 0~065%.

~ 7 -.' ' ' ': , .

~` 1171697 ~itrogen, being one of the inevi-tably entrained im-purities, is generally contained in an amount of not more than 0.010~. If the nitrogen content exceeds 0~0060%, the eventual AlN content increases sufficiently to degrade hot workability.
Thus, the nitrogen content is preferably kept below 000060%.
Phosphorus, another of the ine~ritably entrained impurities, segregates itself in the austenite grain boundary and impairs ductility at elevated temperatures. ~o preclude the degradation of ductility, prevent creep damage and enhance high-temperature ductility and creep duc-tility, the phosphorus content is preferably not more than 0.010%~
~he heat treatment involved in the production of thick plates of steel according to the present invention generally comprises a normalizing step (for example, at 900 to 970 C) and a tempering step (for example, at 650- to 750 C for 0.5 to 10-odd hours).
In situations where the nitrogen content is high (for example, when it exceeds 0.0060%), it is desirable to take any one of the following processes in the course of production for the purpose of ensuring the desired effects of boron and in securing the necessary strength. ~
(1) ~he heating temperature of slabs for the plate rolling is below 1150 C; ~ ~

(2) After the rolllng step, the steel plate is gradually cooled at a rate of no-t more than 30 C/hour;
- (3) ~he normalizlng step is performed twice, wlth the first normalizing operation performed at a higher temperature (950 to 1000 C) and the last one at a regular temperature ~(900 to~970 C).
In order to enhance the hardening effect of boron, .. :

., it is necessary for the nitrogen to be fixed (or com-pounded) with aluminum in the form of Al~ and ~or the boron to be converted to free boron prior to normalizing.
Since both boron and nitrogen are readily segregated in the austenite grain boundary and conversely aluminum is segregated very little, boron and nitrogen which are in a non-equilibrium state tend to be precipitated, partic-ularly when the nitrogen content is high. Further, when the slab heating temperature is above 1150C, Al, B and N
dissolve into the steel during the slab heating process and BN is easy to be precipitated in proportion to fall of the temperature during and after the plate rolling. The reaction of BN + Al + B + AlN can be completed by proceed-ing it at 1000C for 30 minutes or a gradual cooling (lower than 30C/hr.). Therefore, when N content exceeds 0.0060%, by performing any one process of heating the slab at below 1150C, gradual cooling after rolling and high temperature normalizing (900 to 1000C) prior to the regular temperature normali~ing (900 to 970C), nitrogen is fixed with aluminum to be in the form of AlN and boron is converted to free boron so that the desired hardening effect of boron can be obtained.
In a pressure vessel made of the steels of this invention, the hardened portion ln the HAZ can be soft-ened and the creep damage resistivity can be improved by performing a high-temperature PWHT. It is, therefore, desirable to give a high-temperature PWHT (650~ - 720C) to such pressure vessels.
The steels o~ this invention use chromium in an amount not exceeding the specified upper limit to improve the hydrogen attack reslstivity and permit a decrease in the silicon content proportionately to the increase of strength obtained by the chromium. They further use Mn aluminum and boron in the _ g _ ~ .

6~7 respectively specified optimu~ amounts. Consequently, there are obtained very tough Cr-Mo steels which have a high hydrogen attac~ resistivity.
~urther, by limiting the content of nitrogen and phosphorus, among the other inevitably entrained impurities, the AlN content can be decreased, the effect of the boron can be ensured and the hot workability can be improved. ~y lowering the phosphorus content ? the creep rupture resistivity can be heightened. Consequently, there are provided steels which excel in high-temperature ductility and creep ductility and which will endure under harsh use conditions.
Example 1 Steels having the chemical composition as shown in Table 1 were prepared by induction melting, '~hese steels were formed into the plates under the manufacturing conditions shown in ~able 2, i.e. by rolling and treatment after the rolling.
~he mechanical properties of thus formed plates are also shown in Table 2. In ~ables 1 and 2, steel No.1 to No.4 denote respectively the steel produced by the~invention and steel No.5 and No.6 denote respectively the steels which are given for comparison.
As shown in '~able 2, the steels of the invention displayed the enhanced strength even after the high temperature PWH~ and, partlcularly, superior impact property to that of the comparison steels after the step cooling No.1 and No.~ steels containing higher N content were subJected respectively to the different special treatments, as indicated by 3-1 and 3-2 in '~able 2. '~able 2 can be featurlzed by that double normalizing, heating at a lower temperature for rolling and slower cooling 3o after rolling secure the favourable results.

Il. -"`
~ 7 ~he steels with high strength and high ductility , according to the invention can be annealed at a high temperature after welding (PWHT) so that the cree~ damage was effectively prevented.
~able 1. Chemical composition of steels ..____ . ........................... . . .
Stee: _ C Si Mn P Cr Mo Bol. N

steels 1 0~13 0~72 0~49 0~015 1 ~18 Or48 0~0012 0~072 0~0072 infVten tion 20.15 o .57 0 ~ 58 0.012 1 ~ 24 - 59 0 ~ 0005 0 ~ 039 000058 _ 30~12 0~65 0~61 0~013 1 ~45 0~62 0~0006 0~042 0~0062 _ ...
0.15 0 ~ 53 0~ 56 0 ~004 1.40 0~600~0007 0~043 0~0032 _ . _ fOrels 0.16 0.21 0.35 0.015 ~9 0.450. 0033 O ~ 038 0. 0073 com-parison 0.13 0.63 0.560.008 1.22 0.52 _ 0.027 ~.0064 . , : :

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11'716~7 ~xample 2.
14Cr-0.5Mo steel according to the invention and the conventional 14Cr-0.5Mo steel, both of which having respectively the chemical compositions as shown in Ta~le 3, were prepared.
~hese steels were heated at 1230 C and ~alized at 930'C for one hour to investigate the relationship of the tempering parameter with the mechanical properties. ~he results of the investigation are shown in Figure 2. As can be seen from Figure 2, as compared with the conventional steel, the steel of the invention becomes less deteriorated in the strength and impact value in proportion to the rise of the tempering temperature.
In other words~ the steel of the invention is provided with sufficient strength and toughness even by high temperature annealing after welding (PWH~) so as to prevent the creep damage in HAZ.
~able 3. Chemical composition of 1-Cr-1,~Mo steels __._ _ - . .... ...
Steel C Sl Mn Cr Mo ASlol B
.

steel 0.160.65 0.60 1.40 0.60 00025 _ ........ _ ~ _ Steel of the 0.140.65 0.60 1.40 0.55 0.055 0.0006 invention . . -- ~

Variations in the present invention will be apparent to those of ordinary skill in this art and yet still fall within the scope of the appended claims. ~
I ~ .
.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A high strength steel which also displays high hydrogen attack resistivity, good high-temperature ductility and high creep rupture strength, said steel being useful in fabricating pressure vessels, said steel consisting essentially of 0.11 to 0.17% of C, 0.50 to 1.20% of Si, 0.46 to 1.00% of Mn, 1.00 to 1.50% of Cr, 0.45 to 0.65% of Mo, 0.00015 to 0.0015% of B, 0.010 to 0.100% of Sol, Al, 0 to 0.006% of N as an impurity and 0 to 0.010% of P as an impurity, said percentages being by weight, and the balance of Fe.

2. A high strength steel as defined in Claim 1, wherein said Si content is between 0.50 to 0.70%, said Cr content is between 1.25 to 1.50%, said B content is 0.0002 to 0.0009% and said Sol. Al content is 0.035 to 0.065%, said percentages being by weight.

3. A high strength steel as defined in Claim 1, wherein said steel is in the form of a plate having a thickness of at least 50 mm.

4. A steel plate being useful in fabricating pressure vessels, said steel plate having essentially the compo-sition of 0.11 to 0.17% of C, 0.50 to 1.20% of Si, 0.46 to 1.00% of Mn, 1.00 to 1.50% of Cr, 0.45 to 0.65% of Mo, 0.00015 to 0.0015% of B, 0.010 to 0.100% of Sol. A1, 0 to 0.006% of N as an impurity and 0 to 0.010% of P as an impurity, said percentages being by weight, and the balance of Fe and being used after a post weld heat treatment at a temperature within the range of 650°
to 720°C.

5. A steel plate as defined in Claim 4, wherein said Si content is between 0.50 to 0.70%, said Cr content is between 1.25 to 1.50%, said B content is 0.0002 to 0.0009%
and said Sol. Al content is 0.035 to 0.065%, said percent-ages being by weight.