GB2078253A - Phosphorous-containing seawater-resistant steels of improved weldability - Google Patents

Abstract

Phosphorus-containing seawater-resistant steels of improved weldability, have a basic steel composition containing not more than 0.07% C, not more than 1.0% Si, not more than 2.5% Mn, from 0.06 to 0.020% P, not more than 0.005% S, from 0.003 to 0.2% Al and not more than 0.004% N, the balance being iron and perhaps impurities. Modified steels may also include one or more of the following elements, Mo Cu, Ni, Co, W, up to a maximum of 4% in total, with Nb, V, Ti, Zr, Ta, B, in rare earth elements including yttrium, Ca, Mg, Te, and Se in desired amounts up to a total of 0.2% for specific purposes. These steel alloys are economical and reliable steel materials for the fabrication of welded marine and submarine structures, and show excellent corrosion-resistance in high-humidity corrosive environments containing salt, while still displaying excellent weldability.

Description

SPECIFICATION Phosphorus-containing sea-water-resistant steels of improved weldability The present invention relates to low-alloy steels, possessing good welding characteristics as well as high sea-water corrosion-resistance, and which therefore are particularly useful for the fabrication of welded structures to be used in sea-water-containing corrosive environments as well as in other more usual corrosive environments, in which the atmosphere participates, such as bridges.

There are many corrosive environments in which sea-water plays a part, including those which submarine structures are immersed in sea-water, those in which marine structures are alternately splashed with sea-water and immersed in sea-water and those in which sea-going or land-based equipment or material is exposed to a high-humidity and closed corrosive environments such as that to which for example ballast tanks of ships are subjected, as well indeed as many others. Of the justmentioned corrosive environments it is perhaps that to which ballast tanks are exposed which is the most severely corrosive; and there seem to be no conventional corrosion-resistant steels which can be relied upon to be satisfactorily resistive against this corrosive environment, despite all the thought which has been directed to this problem and the attempts which have been vainly made to deal with it.

In the course of normal service ballast tanks are alternately filled with sea-water and emptied, while occasionally they are subjected to temperatures as high as 500C. Thus ballast tanks are usually kept in a closed, high-humidity state; and consequently a very thin film of sea-water is formed on the surfaces of the steel plates which are'components of the ballast tank, and enough oxygen is supplied to the steel surfaces to promote severe local corrosion of the steel plates. In such a situation the corrosion rate can be as high as 1 mm per year or even much higher. So great a degree of corrosion of the ballast tanks will directly and quickly affect the soundness of ships, and this kind of deterioration has long been regarded as a critical problem for shipsafety.

Similarly, bridges built near the sea-shore or over the sea are liable to be severely attacked by the splashing of sea-water. Certain parts, particularly the lower surfaces or the inside surfaces of th- bridge girders, are often exposed to high humidity and a large amount of salt is deposited thereon; and consequently the corrosion suffered by these parts can be several times more severe than that encountered by other parts, and this has for long been recognised as a very significant but still unsolved problem in relation to the duration and safely of marine bridges.

Of course many measures have already been contemplated and tried out in the fight against the corrosion of marine structures. Thus for instance protective coatings have conventionally been applied, such as tar-epoxy paint coatings on ordinary carbon steels; and cathodic corrosion control using Zn anodes has been widely adopted for the portions of structures immersed in sea-water. These measures however do not provide satisfactory corrosion protection -- because the pain coatings have only a short life and must periodically be reapplied, and because as regards cathodic corrosion control not only must the electrodes be periodically replaced but anyway it has absolutely no effect in protecting the portions above sea-level.

For reasons of this kind it has therefore long been generally held that the most reliable and effective way of preventing or limiting corrosion in a sea-water environment must be to build the relevant structures from economical steels having excellent corrosion resistance, provided that they have good weldability.

Among conventionally known corrosion-resistant steels there are some commercially-available sea-water-resistant steels, such as 0.2C - 0.1 P - 0.5Cu - 0.5Ni steels, which are recognised as having the most excellent corrosion resistance; but unfortunately these conventional sea-waterresistant steels have poor weldability, and very often prove susceptible to weld cracks when subjected to ordinary welding. Accordingly these conventional sea-water-resistant steels turn out in practice to be more or less unusable as steel materials for building welded structures, such as in ship building or in other similar applications.

There are other corrosion-resistant steels, such as the one containing 0.01 to 0.3% C, 0.25 to 6.0: Si, 0.07 to 1% P, 0.15 to 5.0% Cu, 0.30 to 2.0% Cr and not more than 0.65% Ni disclosed in Japanese Patent Publication No. Sho 13-4411, and the similar one containing not more than 0.2% C, 0.01 to 1.0% Si, 0.2 to 2.0% Mn, not more than 0.10% p, 0.20 to 0.60% Cu, not more than 1.0% Cr, 0.027 to 0.5% Ti and 0.05 to 0.5% Nb disclosed in Japanese Patent Publication No. Sho 38-8211.

These just-mentioned steels contain chromium, phosphorus and copper, and provide satisfactory resistance against weather and sea-water corrosion; but they show poor corrosion-resistance in an atmosphere containing salt and high humidity, because they have a high carbon content and contain chromium. Furthermore, these conventional steels are not restricted with respect to their nitrogen and sulphur contents and despite their high carbon contents they are of inferior weldability and toughness, and thus cannot in practice be used, at any rate as a thick plate material, in the fabrication of welded structures.

Another somewhat similar but "no-chromium" steel disclosed in Japanese Patent Publication No.

Sho 50-808 contains 0.001 to 0.3% C, 0.01 to 1.0% Si, 0.1 to 1.5% Mn, 0.05 to 0.09% P, 0.15 to 1.0% Cu, 0.1 to 1.0% Ni, and one of the elements Ti, Nb, Zr, V and Mo in an amount of 0.01 to 1.0%. In actual samples of this just mentioned steel the carbon content is high (0.10% or higher) and there is no restriction with respect to its S and N contents, so this steel when used to fabricate large marine structures proves unsatisfactory as regards weldability and toughness, and furthermore does not really display enough corrosion-resistance in atmospheres containing high salt and humidity contents.

There is therefore a strond demand for the development of steels which will offer improved weldability as well as corrosion-resistance as compared with the conventional corrosion-resistant steels, and which will be usable for the fabrication of large welded structures.

in an effort to meet that demand, still other steels have been proposed such as those disclosed in Japanese Laid-Open Patent Applications Nos. Sho 51-71817 and Sho 52-123918. The steel disclosed in Japanese Laid-Open Patent Application No. Sho 51-71817 has a basic composition of Si Mn-P-Cu-Cr-Ni-Al type with addition of subelements such as V, Nb, Ti and Zr and this steel is quite satisfactory as regards its corrosion resistance, but it contains nickel which is really too valuable to be used in this fashion. The steel disclosed in Japanese Laid-Open Patent Application No. Sho 52-123918 additionally comprises Ti and Zr as essential alloying elements beyond the aboveindicated basic composition, and also contains 0.002 to 0.01 5% N.

The steel disclosed in the latter prior art is usable as a weather-resistant steel, which moreover due to the nitrogen addition displays a fair toughness in the heat-affected zone in large-heat-input welding, particularly around weld-bonds. In this steel, improvement of the toughness in weld-metals is anticipated by formation of TiN and ZrN, and for this purpose the nitrogen content in actual samples is maintained as high as 0.0048% minimum.

We ourselves have also proposed a steel usable in ballast tanks etc., as disclosed in Japanese Laid-Open Patent Application No. Sho 53-70911, which has a basic composition of low C-P-Mo type and can provide excellent corrosion-resistance under a closed, high-humidity atmosphere, but which still needs to be improved as regards toughness in the weld-metals in large-heat-input welding.

Against the background of prior art discussed above, we have conducted various and extensive studies into the effects of various alloying elements upon the corrosion ratein corrosive environments and upon the weldability of the resultant steels. What we have found is that in phosphorus-containing steels it is still possible to achieve a relatively high strength and a corrosion-resistance much better than that of the conventional steels, even if the carbon and nitrogen contents are reduced to their lower extremities; and furthermore that the poor weldability which has long been thought to be inherent in a high phosphorus steel containing nitrogen can be overcome-by reducing the carbon, nitrogen and sulphur contents of the steel to their lower extremities.We have been much involved in recent developments of steel-making techniques which have enabled the carbon, nitrogen and sulphur contents of the steel to be lowered, and enabled steels to be easily produced with lowered nitrogen and sulphur contents; and extensive investigations into various steels whose N and S contents have been reduced to their lower extremities have led us to certain conclusions as follows.

Phosphorus has a greater solid solubility in ferrite than in austenite, so that it is uniformly distributed in solid solution in an extremely low carbon steel; and in a steel containing a pearlite structure, embrittlement of the steel due to the phosphorus content becomes more apparent due to the interaction between the cementite and N, P and S during the welding, so that the pearlite portion containing phosphorus and sulphur is very likely to become a site for embrittlement cracks and weld cracks. When however the pearlite portion is reduced it has been found that the tendency for cracks to occur at the interface between the ferrite and the carbide is also reduced; and it is also possible to eliminate the adverse effects of the phosphorus content by reducing the nitrogen and sulphur contents to their lower extremities.

According to this invention there are therefore provided phosphorus-containing low-alloy steels, possessing good welding characteristics as well as high sea-water corrosion-resistance and therefore especially useful for the fabrication of marine and submarine welded structures, which contain the following alloying elements in the following percentage amounts by weight, namely.

C = not greater than 0.07% Si = not greater than 1.0 , Mn = not greater than 2.5% P =0.06% to 0.20% S = not greater than 0.005% Al =0.003% to 0.2% N = not greater than 0.004% the balance being essentially iron, with or without any conventional impurities.

Conventionally carbon is regarded as having no effect on the corrosion-resistance of steel, but this view turns out to be based upon the limited knowledge previously available which came from studies made on steels with carbon contents of not less than about 0.05%. What we have found is that in the case of low-nitrogen steels when their carbon contents are not less than 0.07% these indeed are not effective to enhance the corrosion-resistance of such steels in high-humidity atmospheres containing salts, but the further the carbon content of the steels falls below 0.07% the greater is the improvement in their corrosion-resistance, for the reasons mentioned elsewhere before; and this kind of improvement becomes all the more apparent when the steel contains phosphorus, copper and molybdenum.

Carbon is also commonly recognised as having a substantial tendency to impair weldability, and this is a further reason why the carbon content in the steel alloy compositions of this invention is limited to 0.07% or less.

On the other hand if the carbon content becomes too low it gets increasingly difficult to maintain the strength of the steel, while anyway there are some practical difficulties operating at an industrial level about lowering the carbon content of the steel beyond a certain point. On a balance of considerations we therefore think it is desirable to maintain The carbon content of the steel at not less than 0.01%.

Silicon, although it may have had to be added to the moisten steer for purposes of deoxidation, has an adverse effect on the corrosion-resistance of the steel. The impairment of the corrosion-resistance ot the steel is particularly bad when the silicon content exceeds 1.0%, and it is for this reason that in the steel alloys of this invention the silicon content is limited to 1.0% or less.

On the other hand the presence of some silicon can have a beneficial effect on the strength of the steel, and is sometimes added for that purpose as well as for the purpose of the above-mentioned deoxidation. It may therefore be desirable to ensure that not less than 0.01% of silicon is present.

Manganese has no effect onthe corrosion-resistance of the steel alloy, but may be added to the molten steel for purposes of deoxidation and/or to secure improvements in the hot-workability of the steel. There is moreover some shortage of strength in the extremely low-carbon steels according to the present invention, which may to some extend be made good by the inclusion of manganese. Up to a point it is perhaps true to say that the larger the manganese content the better for these various purposes, but we have found no significant additional effect from the inclusion of manganese in the steel alloys of the present invention beyond an upper limit of 2.5%.

Although not absolutely necessary, some manganese will desirably be present on account of the above-mentioned considerations, and it is therefore desirable that the manganese content of the steel alloy should be not less than a lower limit of 0.2%.

As regards phosphorus, the conventional recommendation has been to maintain the phosphorus content as low as possible, because phosphorus has conventionally been thought to promote the hot cracking of weld-metals, but we have found that the segregated solidification of phosphorus can be very much reduced by taking the carbon, nitrogen and sulphur contents down to their lower limits, and in this way the steel can be substantially completely freed from any risk of cracking of weld-metals.

This may be attributed partially to the fact that the solidification reaction in the Fe-P-low S-C-low N type assumes a iron solidification when the carbon content is not larger than 0.11% and no peritectic reaction is caused. It has however been found that due to the segregation of C, N and S during the solidification of steel ingots, the weldability and the weld-bond toughness can be improved by lowering the carbon, nitrogen and sulphur contents in the molten steel, to respectively not more than 0.07% C, not more than 0.004% N and not more than 0.005% S.When one keeps within these just-stated limits, then even when carbon, manganese etc. segregate, the phosphorus should be uniformly distributed and thus should no longer be able to produce adverse effects on the hot cracking susceptibility of the weldmetal and the heat-affected zone and on the weld-bond toughness.

Phosphorus is recognised as a most effective element to strengthen the ferrite matrix by solidsolution-hardening; and we have found that, provided the adverse effects of phosphorus on the weldabiiity are eliminated, it becomes possible to secure a relatively high strength property by increasing the phosphorus content, regardless of there being an extremely low carbon content.

Moreover, a steel containing a low proportion of pearlite will be free from the local galvanic action associated with the ferrite and pearlite, and furthermore the higher phosphorus content can quite remarkably enhance the corrosion resistance of the ferrite.

For some or all of the above-indicated reasons it has been found that the steel alloy compositions of the present invention, when used to fabricate welded structures, can display satisfactory weldability while having a corrosion resistance much better than that of conventional corrosion-resistant steels.

Thus in summary phosphorus is highly effective to improve the corrosion resistance of the steel in high-humidity atmospheres containing salt; but that advantage is on the other hand offset by the fact that its presence severely impairs the weldability of the steel when it is included in ordinary carbon steels containing 0.1 5% C. The inherent corrosion-resistance of the steel alloy is still further enhanced by the inclusion of phosphorus in extremely low-carbon steels containing not more than 0.07% C, for which purpose the phosphorus must be present in an amount of not less than 0.06%.On the other hand even in the case of steel alloys with lowered carbon contents not greater than 0.04% and lowered sulphur contents not greater than 0.005% we have found that weldability deteriorates unacceptably if phosphorus is present in an amount beyond the upper limit for the phosphorus content in the steel alloys of this invention, namely 0.20%.

As regards sulphur, it is essential that-the sulphur content of the steel alloys of this invention should be kept down to 0.005% or lower, since this low sulphur content can eliminate the adverse effect of the phosphorus upon the weldability and the toughness of the weld-metals, and also makes thE steel less susceptible to weld-cracking, enhances the clearness, and improves its corrosion-resistance.

The interactions of the sulphur on the one hand with the carbon, nitrogen, phosphorus, etc. on the other hand have been described elsewhere herein.

The relationship between the sulphur-content in a steel alloy and its weld-cracking resistance can best be seen graphically. For purposes of illustration reference should be made to the accompanying drawing, which is a graph showing the relationship between the sulphur content and the weld-cracking resistance in respect of a steel otherwise containing 0.04% C, 0.2% Si, 1.2% Mn, 0.07% P, 0.25% Cu, 0.02% Al and 0.0035% N. From this graph it can clearly be seen that the weld-cracking resistance can be greatly improved by lowering the sulphur content.

Therefore, even if the steel is heated into the austenite region and rapidly cooled during the welding, it is not susceptible to cold cracking and embrittlement of weld-metals due to P, N, S, and C, but on the contrary the steel can retain a high degree of toughness in the large-heat-input weld-metals.

Aluminium must be present in the steel alloys of this invention to an extent of at least 0.003% in order to refine the grain and improve the corrosion-resistance of the steel. Indeed we have in particular found that this element is effective to enhance still further the improvement in the corrosion-resistance of the steel brought about by phosphorus, copper, etc., provided however that the amount of the aluminium present does not exceed 0.2%.

For the reasons set forth elsewhere the nitrogen content of the steel of this invention must be restricted to 0.004% or less, and it will then be effective to eliminate the otherwise-adverse effect of the phosphorus content upon the weldability and the toughness of the weld-metals, as well as also effective through intereaction with the low C and low S contents to reduce the sensitivity of the steel to weld cracking.

There is however on the other hand some restriction upon how far the nitrogen content of the steel should be reduced, if only because of the practical difficulties of nitrogen-reduction when operating at an industrial level. Accordingly we much prefer that the nitrogen content of the steel alloy should not be less than 0.001%.

The steel alloys of this invention, as so far described, can be modjfied by the inclusion of further alloying elements, as will be described subsequently herein. To assist in describing these modified steel alloys it is convenient here to classify all the steel alloys (modified as well as unmodified) of this invention into eight distinguishable sub-groups or sub-categories, as illustrated in Table 1 below. TABLE 1 Composition Ranges of Inventive Steels (%)

1 2 3 4 5 6 7 8 C #0.07 #0.07 #0.07 #0.07 #0.07 #0.07 #0.07 #0.07 Si #1.0 #1.0 #1.0 #1.0 #1.0 #1.0 #1.0 #1.0 Mn #2.5 #2.5 #2.5 #2.5 #2.5 #2.5 #2.5 #2.5 P 0.06-0.20 0.06-0.20 0.06-0.20 0.06-0.20 0.06-0.20 0.06-0.20 0.06-0.20 0.06-0.20 S #0.005 #0.005 #0.005 #0.005 #0.005 #0.005 #0.005 #0.005 Al 0.003-0.2 0.003-0.2 0.003-0.2 0.003-0.2 0.003-0.2 0.003-0.2 0.003-0.2 0.003-0.2 N #0.004 #0.004 #0.004 #0.004 #0.004 #0.004 #0.004 #0.004 Mo, Cu, Ni, Co, - at least one at least one at least at least one W in total - - in amount one in - in amount amount#4.0 #4.0 amount #4.0 #4.0 Nb, V, Ti, at least one at least one at least one at least one Zr, Ta, B - - in total - in amount - in amount in amount amount#0.2 #0.2 #0.2 #0.2 Rare Earth at least at least at least at least Element - - - one in - one in one in one in Ca, Mg, Te, Se amount #0.2 amount #0.2 amount #0.2 amount #0.2 As appears from sub-groups 2, 5, 6 and 8 in Table 1 above, the elements molybdenum (Mo), copper (Cu), nickel (Ni), cobalt (Co) and tungsten (W) may all be included and can have the effect of improving the corrosion-resistance of the steel alloy.Of these elements, copper and nickel tend to increase the resistance of the steel to general corrosion, while molybdenum, cobalt and tungsten tend to contribute some increase in the resistance of the steel to localize corrosion. In order to produce their desired effects these elements may be added to the basic steel composition either singly or in any desired combination.

Generally-speaking the beneficial effects of the just-mentioned elements will increase as greater amounts are added, but only up to a point - at and beyond an upper limit of 4.0% of such elements overall (i.e. no matter whether present singly or in any combination) the beneficial effects upon corrosion-resistance seem to reach saturation, but there begin to appear certain adverse effects upon the weldability and toughness of the steel.

It is accordingly a preferred feature of this invention to provide steel alloys as described elsewhere herein but modified by the inclusion therein of molybdenum and/or copper and/or nickel and/or cobalt and/or tungsten in an overall amount of such element(s) not exceeding 4.0%.

When these elements are added in combination, it is desirable that the steel alloy should contain from 0.05 to 1.0% Mo, from 0.02 to 1.0% Cu and most preferably from 0.02 to 0.40% Cu, from 0.05 to 2.0% Ni, from 0.01 to 1.0% Co and from 0.01 to 1.0% W.

As appears from sub-groups 3, 5, 7 and 8 in Table 1 above, the elements niobium (Nb), vanadium (V), titanium (Ti), zirconium (Zr), tantalum (Ta) and boron (B) may all be included and when present in amounts not greater than 0.2% overall (i.e. mo matter whether present singly or in combination) of the basic steel alloy composition will improve its tensile strength without impairing its weldability, thus extending the possible fields of application of the steel. If however these elements are present in amounts greater than 0.2%, they will significantly impair its toughness and weldability.

According to another preferred feature of this invention here are also provided steel alloys as described elsewhere herein but modified by the inclusion therein of niobium and/or vanadium and/or titaniurii'åiid/or zirconium and/or tantalum and/or boron in an overall amount of such element(s) not exceeding 0.2%.

Some of these elements also have additional effects. Thus Nb, V and Ti fix the carbon in the steel as carbides, thereby promoting a reduction in the carbon content and thus further enhancing the effect of the addition of phosphorus. Again Ti, Zr and Ta are effective to reduce or modify sulphide inclusions, in phosphorus-containing steels in particular, and they are also effective to improve the toughness of the weld-heat-affected zones. These additional effects of these elements are more apparent when they are added in appropriate amounts, either singly or in combination, as described hereinafter.

In some corrosive environments, as sometimes encountered for instance in oil tanks and oil pipe lines, hydrogen deriving from H2S corrosion reactions penetrates into the steel and builds up in non metallic inclusions, particularly sulphide inclusions, to cause corrosion cracking.

There are other elements which, just like titanium and zirconium, are effective to reduce or modify the non-metallic inclusions, particularly the sulphide in the steel. As appears from sub-groups 4, 6, 7 and 8 in Table 1 above, these other elements, namely the rare earth elements (here indentified by the symbol RE) as herein defined and calcium (Ca), magnesium (Mg), tellurium (Te) and selenium (Se) can therefore be included in the basic steel composition of the present invention to provide effective corrosion-resistance even in corrosive environments in which hydrogen is present. The term "rare earth elements" is used herein to indicate one or more of the elements having an atomic number from 57 to 71 together also with yttrium (Y).

However, although up to a point the greater the amount present of those just-mentioned alloying elements the greater their effect on the improvement of the corrosion-resistance of the steel, when the amount of such elements present reaches 0.2% overall (i.e. no matter whether present singly or in combination) any further increase seems to result in no further improvement in corrosion-resistance but on the contrary some lowering of the toughness of the steel. The additions of such element(s) is therefore restricted to 0.2% maximum overall.

According to yet another preferred feature of this invention there are also provided steel alloys as described previously herein but modified by the inclusion therein of one or mordi rare earth elements as herein defined and/or calcium and/or magnesium and/or tellurium and/or selenium in an overall amount of such element(s) not exceeding 0.2%.

From what has already been said hereinbefore it should be clear that the steel alloys of this invention may be modified in several different ways; and indeed they may be modified simultaneously in more than one of these ways, subject only the reservation that the overall amount of all these alloying elements must be kept within bounds. Although the addition of such alloying elements will up to a point further improve the weldability, the strength and the corrosion-resistance of the steel, when the total overall addition of such alloying elements exceeds 0.2% there seems to be on additional improvement in these qualities, but on the contrary some deterioration in other material qualities, such as its toughness.

According to a yet further preferred feature of this invention there are also provided steel alloys as previously described herein which however have been modified by the inclusion therein as alloying elements of: - (A) niobium and/or vanadium and/or titanium and/or zirconium and/or tantalum and/or boron; as well as - (B) one qr more rare earth element(s) as herein defined and/or calcium and/or magnesium and/or tellurium and/or selenium; said (A) and (B) alloying elements being present in an overall amount not exceeding 0.2%.

When two or more of the above elements are added in combination, it is desirably that the steel alloy should contain from 0.01 to 0.07% Nb, from 0.01 to 0.07% V, from 0.003 to 0.03% Ti, from 0.003 to 0.08% Zr, from 0.003 to 0.07% Ta, not more than 0.01% B, not more than 0.10/0 RE, not more than 0.1% Ca, not more than 0.05% Mg, from 0.01 to 0.08% Te and from 0.1 to 0.08% Se.

All of the basic steel alloy and the modified steel alloy compositions according to the present invention can be easily manufactured by ordinary iron-making steel-making and rolling methods; and the above-described desirable properties of the steels of the present invention can be secured in the "as-rolled" condition, while these properties can be further improved by ordinary quenching, tempering or normalizing operations after the rolling.

In order still further to clarify this invention it will now be illustrated with reference to certain preferred embodiments as follows:- The chemical compositions of various test pieces are shown in Table 2.

TABLE 2

Chemical compositions (%) Steels C Si Mn P S Al N Mo Cu Ni Co 1 1 0.14 0.28 1.31 0.02 0.013 0.01 0.006 - - - o ." 2 0.12 0.26 1;22 0.025 0.015 0.02 0.005 0 - - 3 3 0.09 0.29 1.12 0.018 0.009 0.02 0.006 0 - - E o 4 0.03 0.39 1.37 0.15 0.010 0.02 0.008 0.10 0.36 - ~ .

5 0.040 0.20 1.20 0.07 0.003 0.02 0.0038 - - - 6 0.023 0.25 1.20 0.07 0.004 0.03 0.0032 - - - 7 0.020 0.40 1.30 0.09 0.005 0.08 0.0034 -'- - - 8 0.032- 0.33 1.25 0.09 0.002 0.03 0.0028 - 0.34 - 9 0.031 0.19 1.38 0.07 0.003 0.06 0.0030 0.22 0.31 - 10 0.030 0.33 1.40 0.08 0.003 0.08 0.0035 - 0.36 - 0.10 11 0.038 0.30 1.41 0.10 0.003 0.02 0.0038 0.13 0.33 0.20 12 0.030 0.41 1.42 0.10 0.002 0.02 0.0036 - - - '13 0.032 0.30 1.40 0.97 0.004 0.02 0.0029 - - - 14 0.028 0.43 1.50 0.09 0.005 0.06 0.0024 - - - 15 0.032 0.38 1.50 0.09 0.003 0.10 0.0026 - - - 16 0.038 0.28 1.31 '0.06 0.004 0.02 0.0029 - - - 17 17 0.041 0.31 1.29 0.04 0.003 0.04 0.0033 - - - a) cho 18 0.020 0.35 1.25 0.11 0.003 0.06 0.0032 - - - 19 0.030 0.32 1.05 0,06 0.003 0.05 0.0026 - 0.25 - E 20 0.033 0.28 1.03 0.07 0.004 0.06 0.0028 '0.15 0.27 - a) 21 0.045 0.25 1.03 0.09 0.003 0.07 0.0029 - 0;;30 0.12 22 22 0.021 0.35 0.90 0.12 0.004 0.05 0.0031 - 0.35 - 0.05 a) 23 23 0.030 0.33 0.85 0.06 0.004 0.06 0.0032 - - 0.28 - L 24 0.031 '0.36 0.80 0.09 0.003 0.04 0.0027 0.05 .0.25 - 25 0.025 0.33 0.79 0.12 0.004 0.05 0.0025 - 0.32 - 0.04 26 0.020 0.28 0.90 0.11 0.003 0.05 0.0030 - 0,32 0.20 27 0.036. 0.48 t.33 0.07 0.003 0.05 0.0020 - - - 28 0.032 0.38 1.42 0.08 0.002 0.08 0.0035 - - - 29 0.033 0.40 1.38 0.07 0.002 0.06- 0.0036 - - - 30 0.034 0.42 1.40 0.08 0.003 0.04 0.0032 0 ::0032 31 0.032 '0.48 1.43 0.10 0.003 0.03 0.0033 0.10 0.20 0.10 32 0.028 0.28 1.42 0.09 0.002 0.04 0.0035 - - t).33 0.20 0.05 33 0.020 0.36 1.42 0.07 0.002 0.08 0.0036 - 0.30 0.20 34 0.020 0.40 1.40 0.07 0.003 0.10 0.0036 0.10 0.33 - 0.03 35 0.030 0.42 1.40 0.09 0.003 0.10 0.0038 0.20 0.33 0.20 36 0.029 0.45 1.40 0.09 0.002 0.09 0.0037 0.10 0.20 0.10 TABLE 2 (cont.)

Chemical Compositions (%) Corrosion Resistant Steels Nb V Ti Zr Ta B RE Ca Mg Te Se Properties c 1 - - - - - - - - - - - 100 o 2 ~ 2 - - - - - - - - - - - 96 Cd .3 - - - - - - - - - - - 98 O 4 - - - - - - - - - - - 45 0 5 - - - - - - - - - - - 50 6 - - - - - - - - - - - 48 7 - - - - - - - - - - - 47 8 - - - - - - - - - - - 40 9 - - - - - - - - - - - 38 10 - - - - - - - - - - - - 40 11 - - - - - - - - - - - 37 12 0.02 - - 0.005 - 0.001 - - - - - 40 13 0.02 0.02 0.007 - - - - - - - - 46 14 0.05 - 0.01 - - - - - - - - 45 15 - - - '0.01 0.015 - - - - - - 47 16 - - - - - - 0.005 0.005 - - - 51 17 - - - - - - 0.01 - 0.003 - - 49 18 - - - - - - - 0.005 - 0.003 0.005 48 a) 19 0.03 - 0.008 - - - - - ^ - - - 43 0 20 0.02 0.02 - 0.007 - - - - ' - - - 40 a) 21 - - 0.01 0.005 - - - - - - - 39 a) g 22 - - 0.006 - 0.013 0.001 - - - - - 37 23 - - - - - - 0.007 - 0.002 - - 44 24 - - - - - - 0.005 0.005 - - - 40 a) X 25 - - - - - - - - 0.002 0.008 - 38 26 - - ,- - - - - - 0.005 - - 0.003 36 27 0.03 0.02 - 0.005 - - 0.01 - - - - 52 28 0.04 - - 0.008 - - 0.001 - 0.005 0.005 - - 49 29 - 0.04 0.01 - - - - - - 0.01 0.01 50 30 - - - 0.01 0.02 0.001 - 0.01 - - - 49 31 0.02 - 0.005 - - 0.001 - - - - - 38 32 - 0.04 0.008-0.004 -- - 0.005 - - - - 39 33 0.03 - - 0.01 - - 0.005 0.008 - - - 40 34 - 0.02 - 0.01 0.017 - - - 0.005 0.005 - 40 35 0.04 - 0.008 - - - - -0.005 - - 0.01 36 36 - 0.02 0.005 0.01 - 0.001 - - - 0.01 - 40 * One year test resulits in ballast tank.

In Table 2, test pieces No 1 to No. 4 represent steels outside this invention, tested for purpose of comparison and test pieces No. 5 to No. 36 represent the steels according to the present invention.

Table 2 also shows the corrosion-resistance of the steel specimen determined by a one-year test in an actual tanker ballast-tank, constituting a high-humidity corrosive environment affected by seawater, and expressed as a percentage of the corrosion weight-loss of a comparative ordinary steel (Si Mn steel = 100%).

Various properties of the test pieces whose composition is identified in Table 2 were tested, and are set out in Table 3 below.

TABLE 3

y-Groove * Crack ** Yield Tensile Elongation Arresting Simulated Point Strength (GL50) vEo Temp. Weld HAZ Steets (kg/m2) (kg/m2) (%) (kg-m) ("C) vEo (kg-m) Remarks 1 1 32 53 41 7.5 100 3.3 0 2 2 31 51 40 7.2 100 3.5 Cd 3 3 35 53 39 17.9 75 4.5 o" 4 28 46 38 6.7 50 3.6 5 30 41 42 26.8 10 10.8 6 28 42 40 29.8 10 13.8 7 26 43 39 30.0 10 11.5 8 28 43 43 22.0 10 10.0 9 30 45 40 24.8 10 12.8 10 33 50 42 26.8 10 12.9 11 35 51 40 20.0 10 11.2 12 30 47 42 20.3 10 12.5 13 29 47 41 20.0 10 12.0 14 29 50 38 20.0 1G 12.8 15 31 46 40 24.0 10 12.0 16 27 46 38 20.3 10 13.0 , 17 29 48 39 21.5 10 14.0 a) co 18 26 47 41 18.9 10 12,5 a).

.2 19 28 46 40 19.0 10 13.4 20 33 49 38 19.5 10 12.5 Z 21 30 48 40 23.0 10 11.5 ' 22 34 49 39 23.5 10 10.8 Co 23 25 44 41 24.0 10 14.0 24 26 43 43 23.0 10 13.1 25 29 44 42 24.5 10 12.8 26 28 45 41 25.0 10 13.3 27 28 47 40 20.0 10 12.0 28 29 50 39 25.7 10 13.5 29 30 51 40 21.0 10 12.0 30 32 54 40 23.1 10 12.0 31 34 54 38 22.5 10 10.8 32 34 56 40 23.0 10 13.8 33 32 54 40 24.3 10 10.9 34 34 '55 40 22.7 10 10.8 35 37 58 40 22.9 10 12.4 36 33 50 43 23.1 10 12.5 * JIS Z3158 ** 1350 C heat, 80sec from 800 to 500 C *** Controlled rolled.

From an examination of the information set out in Tables 2 and 3, it can be clearly seen that the test piece No. 4 containing P, Cu and Mo and with a reduced carbon content shows quite satisfactory sea-water resistance; but, due to its high N content and excessive P and S contents, the value of vEo (Charpy Impact at 0 CJ of the similated heat-affected zone, which is indicative of the weldability with large-heat-input, is low as shown ii Table 3.

That may be contrasted with the fact that all of the steel compositions with lowered C, N and S contents according to the present invention show a high corrosion-resistance, a low preheating temperature for arresting weld-cracking, and a high toughness in the simulated heat-affected zone, thus displaying excellent properties as a corrosion-resistant steel with high weldability.

It may also be noted that the test pieces Nos. to 11, Nos. 19 to 22, Nos, 23 to 26 and Nos. 31 to 36 all show particularly excellent corrosion-resistance, due to the inclusion of Mo, Cu, N, Co and W therein.

From the foregoing description It should by now be clear that the steel alloy compositions according to the present invention combine a very excellent corrosion-resistance in high-humidity corrosive atmospheres containing salt with good weldability and yet can be very easily produced.

Claims (14)

1, Phosphorus-containing low-alloy steels which contain the following alloying elements in the following precentage amounts by weight, namely: C = not greater than 0.07% Si = not greater than 1.0% MN = not greater than 2.5% P = 0.069/0 to 0.20% S = not greater than 0.005% Al =0.003% to 0.2% N not greater than 0.004% the balance being essentially iron, with or without any conventional impurities.

2. Low-alloy steels as claimed in claim 1, wherein the carbon content of the steel is not less than 0.01%.

3. Low-alloy steels as claimed in claim 1 or claim 2, wherein the silicon content of the steel is not less than 0.01%.

4. Low-alloy steels as claimed in any of the preceding claims, wherein the manganese content of the steel alloy is not less than 0.2%.

5. Low-alloy steels as claimed in any of the preceding claims, wherein the nitrogen content of the steel alloy is not less than 0.001%.

6. Low-alloy steels as claimed in any of the preceding claims, modified by the inclusion therein of molybdenum and/or copper and/or nickel and/or cobalt and/or tungsten in an overall amount of such element(s) not exceeding 4.0%.

7. Low-alloy steels as claimed in claim 6, which contain from 0.05 to 1.0% Mo and/or from 0.02 to 1.0% Cu and/or from 0.05 to 2.0% Ni and/or from 0.01 to 1.0% Co and/or from 0.01 to 1.0% W.

8. Low-alloy steels as claimed in claim 6 or claim 7, which contain from 0.02 to 0.4% Cu.

9. Low-alloy steels as claimed in any of the preceding claims, modified by the inclusion therein of niobium and/or vanadium and/or titanium and/or zirconium and/or tantalum and/or boron in an overall amount of such element(s) not exceding 0.2%.

10. Low-alloy steels as claimed in any of the preceding claims, modified by the inclusion therein of one or more rare earth elements as herein defined and/or calcium and/or magnesium and/or tellurium and/or selenium in an overall amount of such element(s) not exceeding 0.2%.

11. Low-ailoy steels as claimed in any of the preceding claims, modified by the'inclusion therein as alloying elements of: (A) niobium and/or vanadium and/or titanium and/or zirconium and/or tantalum and/or boron; as well as - (B) one or more rare earth element(s) as herein defined and/or calcium and magnesium and/or tellurium and/or selenium; said (A) and (B) alloying elements being present in an overall amount not exceeding 0.2%.

12. Low-alloy steels as claimed in claim 11, wherein the steel alloy contains (A) from 0.01 to 0.07% Nb and/or from 0.01 to0.O7% V and/or from 0.003 to 0.03% Ti and/or from 0.003 to 0.08% Zr and/or from 0.003 to 0.07% Ta and/or not more than 0.01% B, as well as (B) not more than 0.1% RE and/or not more than 0.1% Ca and/or not more than 0.05% Mg and/or from 0.01 to 0.08% Te and/or from 0.1 to 0.08% Se.

13. Low-alloy steels as claimed in any of the preceding claims and substantially as herein described.

14. Low-alloy steels substantially as herein described with reference to Table 1.

1 5. Low-alloy steels substantially as herein described with reference to any of steels 5 to 36 in Table 2.