GB1601651A - Niobiumcontaining weldable structural steel - Google Patents

Description

(54) NIOBIUM-CONTAINING WELDABLE STRUCTURAL STEEL (71) We, KOBE STEEL LTD., a corporation organised under the laws of Japan of 3-18, 1-chome, Wakinohamacho, Fukiai-ku, Kobe-city, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to niobium-containing weldable structural steel having good weldability. More specifically, the present invention relates to a niobium-containing weldable steel having a yield strength of 40 - 70 kg/mm2 prepared by providing a niobium-containing steel with a predetermined composition, so as to improve the toughness in the weld heat-affected zone and resistance to weld cracking.

When added in a small amount of steel, niobium improves the strength and toughness of the steel, and is an economically advantageous element. It is for this reason that a niobium-containing steel (hereinafter referred to as an "Nb-containing steel") has gained a wide application as a weldable structural steel e.g. in pipe-lines, shipbuilding, pressure containers and bridges. Among numerous applications, of Nb-containing steels the detailed description of this specification refers particularly to steels which are used in large quantities to construct pipe-lines. Nb-containing, non-quenched tempered high-tensile strength steel has conventionally been used in large quantities in pipe-lines for transporting crude oil and natural gas. However, the pipe-lines that have so far been laid down have a small pipe diameter and low inner pressure, and are used at a relatively high temperature which is not lower than about 0 C. Hence, the requirement for toughness in the weld heat-affected zone has not previously been highly demanding.

However, the laying of pipe-lines on a large scale has gradually become concentrated in ultra-cold regions of the U.S.S.R., Canada and the U.S.A., and the material transported has also changed gradually from crude oil to natural gas. An unexpected problem has come to the fore in that the conventional Nb-containing steel for pipe-lines suffers from embrittlement of the weld heat-affected zone (HAZ) in the seam weld'portion at the time of steel making and, hence fails to ensure sufficient toughness to withstand use in such ultra-cold regions. As counter measures, on the one hand a multiple-electrode submerged are welding process with a restricted low weld heat input and an MIG welding process have been examined from the viewpoint of the welding process itself in an effort to mitigate the influence of the welding heat on the steel pipe, for example. On the other hand,- the steel material which is used has been studied with a view to obtaining a steel having a composition which does not cause degradation of the weld heat-affected zone. With regard to the properties of the steel material especially, the most desirable steel is one which does not cause embrittlement of the weld heat-affected zone and yet exhibits good notch toughness, even when welding of a large heat input weld, e.g. as in one-side weldin or two-side welding with one or two layers (not more than three passes in each weld groove is carried out, in order to improve welding efficiency and to reduce the cost of production of the pipe. At the same time, the steel must satisfy a specific requirement of a low weld crack sensitivity for the purpose of preventing cracks of the weld portion because, when on-the-site welding is effected in a cold zone, a high cellulose type electrode with a high hydrogen content is generally employed without preheating of the pipe in the low temperature atmosphere.

It has been a pressing need for those concerned in the art to develop a steel strip for a pipe-line which simultaneously satisfies the above-mentioned requirements, i.e., has excellent toughness in the weld-heat affected zone, is less hardenable than previously and has a high resistance to weld cracking.

Low alloy, niobium-containing steels are described in the following U.S. Patents, namely U.S.P. No. 3,592,663; U,S.P. No. 3,619,303; U.S.P. No. 3,721,587 and U.S.P. No.

3,807,990, which represent prior proposals in dealing with the aforementioned or similar problems.

We have attempted to resolve in improved manner the problems involved in the welding of Nb-containing steel, and to provide an Nb-containing weldable structural steel, especially an Nb-containing steel for use in pipe-lines, having an acceptable toughness in the weld heat-affected zone and an improved resistance to weld cracking.

It is well known that, in general when welding involving a high weld heat input, e.g.

automatic welding, is applied to a weldable structural steel the weld heat-affected zone, especially the weld bond of the joint and the portions near the bond, are embrittled. To prevent this embrittlement, it is believed effective to convert the structure of the heat-affected zone to fine ferrite, pearlite, lower bainite or a mixed structure of lower bainite and martensite. However, such a treatment is not an effective expedient in the case of the Nb-containing steels of the present invention.

Thus, the structure formed in the weld heat-affected zone varies in accordance with the hardenability of the steel. When a ferrite pearlite structure is desired, for instance, hardenability must not be so high as inevitably to restrict to the amounts of the alloy elements to be added. Hence, this treatment is applicable only to a steel consisting principally of an Si-Mn system with a yield strength in the 20-40 kg/mm2 class and having a small amount of alloy addition. If the treatment is applied to an Nb-containing steel, there is obtained an upper bainite structure having a very much inferior toughness and in which, contrary to the desired result, the embrittlement of the heat affected zone is promoted.

In order to obtain the lower bainite structure or the mixed structure of the lower bainite and martensite, on the other hand, it is necessary to use a steel incorporating expensive elements such as Ni, Cr, and Mo, for example in large quantities. Further, the steel containing large amounts of these elements has a very much inferior resistance to weld cracking and its production involves a drastic increase in the cost of production. For these reasons, it is difficult to use a steel of this type as a weldable structural steel.

In the Nb-containing steel, also the weld bond of the joint and of the portions near the bond are rapidly heated to about 1,300 C. or above at the time of welding. Consequently Nb-carbonitrides that have precipitated during rolling of the product or after quenching and tempering are thermally decomposed by the welding heat and the products of decomposition are dissolved in the matrix thus causing significant increases in hardenability, hardening of the bond and adjoining regions and deterioration of the resistance to weld cracking. Hardening of the heat-affected zone can therefore be theoretically prevented by minimizing redissolution of Nb. However, this is contradictory to, and spoils, an essential feature of an Nb-containing steel.

We have made intensive studies in an attempt to find the cause of deterioration of toughness in the heat-affected zone, and to establish means for preventing such deterioration and a method of improving the resistance to weld cracking of an Nb-containing steel while making the most of the feature of an Nb-containing steel that it provides high strength at a cheap cost, in order to adapt the steel to a weldable structural steel, especially to a steel for a pipe-line.

As a result, we determined that the mass-like or spherical structures, or so-called "martensite islands", which occur in the upper bainite structure formed in the weld heat-affected zone at the time of welding of a large heat input weld e.g. in one-side welding with one welding pass or in two-side welding with one welding pass in each groove, are the point of occurrence of the traltsmission route of brittle cracking, and that, when the quantity of the martensite islands exceeds 15%, the toughness of the steel has very much deteriorated. Accordingly, we endeavoured to obtain a steel composition which mitigates the adverse influence arising from the formation of the martensite islands and which improves the resistance to weld cracking.

In accordance with this invention, we have found that the amount of the martensite islands can be restricted to not more than 15% and the weldability can also be remarkably improved by ensuring that the proportions of C, Si, Mn, Nb, Ti and Ab, are restricted to specific ranges as well as by ensuring that the amounts of C, Ti and N have a specific interrelationship. The present invention has been completed on the basis of these findings.

According to one aspect of the present invention, we provide an Nb-containing weldable structural steel having good weldability which consists of 0.005 - 0,04% of C, 0.01 - 0.50% of Si, 1.20 - 2.50% of Mn, 0.01 - 0.07% of Nb, 0.005 - 0.030% of Ti, 0.005 - 0.06% of At and N in amounts satisfying the relations: [C (%) + 10N (%)this not greater than 0.10% and Ti(%)/[C (%) + 10N (%) n is from 0.05 to 0.60, the balance being iron and inevitable impurities whereby the amount of the martensite island in the weld heat-affected zone is restricted to not more than 15% in terms of the area fraction.

According to a second aspect of the present invention, we provide a Nb-containing weldable structural steel having good weldability which consists of 0.005 - 0.04% of C, 0.01 - 0.50% of Si, 1.20 - 2.50% of Mn, 0.01 - 0.07% of Nb, 0.005 - 0.030% of Ti, 0.005 - 0.06% of At, at least one element in the specified amount selected from the group consisting of up to 0.50% of Cu, up to 1.50% of Ni, up to 0.50% of Cr, up to 0.60% of Mo, up to 0.10% of V, up to 0.003% of B, up to 0.02% of Ce and up to 0.003% of Ca and N in amounts satisfying the relations: [C (%) + 10N (%)j is not greater than 0.10% and Ti (%) / [C (%) + 10N (%)j is from 0.05 to 0.60%, the balance being iron and inevitable impurities whereby the amount of the martensite island in the weld heat-affected zone is restricted to not more than 15% in terms of the area fractions.

The Nb-containing weldable structural steels of the present invention, as just defined in the two aspects of the present invention are particularly suited to the construction of pipe-lines although not restricted thereto.

It is possible in accordance with the present invention to provide an Nb-containing, weldable structural steel in which the weld heat-affected zone has a high level of toughness and a high resistance to weld cracking during welding of a large heat input weld as in automatic or semiautomatic welding, including one side welding with one welding pass, two-side welding with two layers, and circular seam welding, and to guarantee a yield strength in a range as high as 40 to 70 kg/mm2.

An explanation will now be given in detail of why the components of the Nb-containing steel of the present invention are stipulated to be in specific ranges.

In cooperation with Ti and N, as described below, a reduced C content restricts the formation of the martensite island, enhances the toughness in the weld heat-affected zone and allows the Nb-containing steel of the present invention fully to exhibit its features.

Accordingly, especially careful attention must be paid to the addition amount of C. For improving the toughness and resistance to weld cracking by reducing the quantity of the martensite island occurring in the weld heat affected zone, the amount of C must be lowered to a value not greater than 0.04%. In this manner, it is possible to restrict the amount of the martensite island to not more than 15% of restricting also the C amount in interrelations with the amount of Ti and N present, as described below. Although the carbon content is preferably as small as possible, there is a problem of production cost in order to reduce it to less than 0.005% in a practical steel. In practice, therefore, the carbon content is in the range of from 0.005 to 0.04%.

Furthermore, it is necessary to restrict the C content in conjunction with the Ti and N contents. If the C content is not higher than the above-mentioned upper limit of 0;04% or in the practical range of from 0.005 to 0.04%, it is difficult perfectly to eliminate the undesirable influence of the martensite island over the toughness in the weld heat-affected zone. In addition to these limitations, therefore, the C content in conjunction with N, that is [C (arc) + 10N (50)], must be restricted to a value not higher than 0.10% and in conjunction with Ti and N, the parameter: Ti(%) / [C(%) + 10N (%)] must be in range of from 0.05 to 0.60. These limitations furnish the heat-affected zone with an excellent impact value.

Ti has the effects of reducing the quantity of the martensite island which is formed in the weld heat-affected zone and, further, of promoting the toughness in said zone, together with the effect of preventing coarsening of an austenite grain size in the heat-affected zone, especially at the weld bond of the joint and in the regions adjoining the bond. If the Ti content is less than 0.005% the effect of TiN in preventing the growth in size of the austenitic grains becomes insufficient, and it also becomes difficult to fix the deterimental free nitrogen or to render it innocuous. Desirably, therefore, at least 0.005% of Ti is to be added. On the other hand, the addition of an excessive amount of it is not desirable because it causes coarsening of TiN in the steel or the formation of large Ti-type inclusions, and adversely affects, the toughness not only of the heat-affected zone but also of the base metal. Accordingly, the upper limit of the Ti content is 0.030%.

It is also an essential requirement of the present invention that, as mentioned above, the Ti content be restricted in conjunction with C and N. Namely, the value of Ti(%)/ C(%) + 10N(%)j ratio must be in the range of from 0.05 to 0.60 under the condition of C(%) + 10N (%) S 0.10%. This constitutes one of the features of the present invention for ensuring that the amount of the martensite island occurring in the weld heat-affected zone is restricted to not more than 15%. If the ratio Ti (%)/[C(%) + 10N(%)] is less than 0.05, it is difficult sufficiently to reduce the amount of the martensite island, and, if the latter ratio exceeds 0.60, on the other hand, the above-mentioned adverse influences of Ti take place and adversely affect the toughness of the heat-affected zone. For these reasons, the Ti content must be in the range-of from 0.005 to 0.030% and at the same time, the ratio: Ti/[C(%) + 10N(%)l must be in the range of 0.05:1 to 0.60:1 [where the value C(%) + 10N (%) is restricted to not more thaji 0.l0% as mentioned above].

Nb is a key element in the steels of the present invention in other words, the steels of the present invention are so-called Nb-containing steels. "Nb is extremely effective in improving the strength and toughness of a steel and, moreover, is an economical element.

The effect of the addition of Nb increases with increasing amounts of the addition but, the toughness in the weld heat-affected zone and the resistance to weld cracking tend gradually to deteriorate. Further, the addition of a large amount of Nb is not economical. For these reasons, the upper limit of the amount is 0.07% while the lower limit is 0.01% because the features of the Nb-containing steel cannot be sufficiently obtained if the addition amount is too small.

As already mentioned, N is an element which has a remarkable effect on the toughness in the weld heat-affected zone in the same way as C does. The range of N content is determined in conjunction with C apd Ti contents. Namely, the N content must satisfy the following two requirements: C (%) + 10N (%) 6 0.10% (1) 0.05 S Ti (%) / [C(%) + 10N(%)] S 0.60 (2) If the carbon content is the practical lower limit of 0.005% and the Ti content is the upper limit of 0.03%, for example, the N content must be lower than 0.0095% (a) from the formula (1) and 0.0045 - 0.0095% ( ) from the formula (2). Hence, the N content must be from 0.0045 to 0.0095% which simultaneously satisfies both (a) and (p). On the other hand, if the C content is its upper limit of 0.04% and the Ti content is its lower limit of 0.005%, the N content is not greater than 0.006% (y) from the formula (1) and not greater than 0.006% (o) from the formula (2). Hence, the N content in this case is stipulated to be not greater than 0.006% which simultaneously satifies both (y) and (5).

Si secures the strength of the base metal and is effective as a deoxidizer in steel making.

For these purposes, 0.01 - 0.50% of Si is added.

As in the case of Si, Mn is added in order to provide the steel with a required strength. If the amount is less than 1.2% it is difficult to obtain a yield strength of at least 40 kg/mm2 in the ultra-low carbon-type steel of the present invention. Accordingly, at least 1.2% of Mn is preferably added. If Mn is added in excess, however, Mn segregation is promoted in the steel ingot, thereby not only causing the cleanliness of the steel to deteriorate but also facilitating the formation of the martensite island in the weld heat-affected zone, thus increasing hardenability and degrading the toughness and resistance to weld the cracking.

Hence, the upper limit of the Mn content is not greater than 2.5%.

At is effective as a deoxidizing element during steel-making and also as a grain refining element. It also functions as a nitride-forming element and fixes the free nitrogen formed in the weld-heat-affected zone and exhibits its effect for stabilizing and improving the toughness in the weld heat-affected zone. However, the addition of At in excess is not desirable because it causes an increase in alumina-type inclusions and lowers the cleanliness of the steel. At is therefore added in an amount in the range of from 0.005 to 0.06%.

In addition to the above mentioned elements, the Nb-containing steel in accordance with the present invention may further incorporate, if necessary, solid solution elements such as Cu, Ni, Cr and Mo and trace elements such as V, B, Ca and Ce in proper amounts, in order further to improve the toughness and other various properties such as strength. Needless to say, however, these additional elements must be added within the ranges which do not lead to a deterioration in the toughness of the weld heat affected zone and in resistance to weld cracking. A predetermined limitation is further imposed on the amount of each element to be added by any peculiar effect of the element on the properties of the steel strip such as strength and toughness, and also from the aspect of production technique.

Cu increases the strength without exerting an adverse effect on the toughness of the base metal and the weld heat-affected zone, and improves resistance to hydrogen-induced cracking and corrosion resistance. However, the upper limit is set at 0.50% because, if the amount of Cu exceeds 0.50%, cracking tends to occur on the surface of the steel strip during rolling.

Although Ni has the effect of remarkably improving the toughness of the base metal and the weld heat-affected zone, the addition of Ni in excess is not desirable for a weldable structure where stress corrosion cracking is a serious problem, and invites an increase in the cost of production. It is, therefore, desirable that Ni be added in an amount not greater than 1.50%.

Cr is a useful element for securing the strength of the base metal. However, the addition of Cr in excess amounts causes hardening of the weld heat-affected zone and a deterioration in the resistance to weld cracking. For this reason, Cr is preferably added in an amount not greater than 0.50%.

Mo also is a useful element for maintaining the strength of the base metal. If added in excess however, Mo increases the amount of the martensite island, lowers the toughness of the weld heat-affected zone and enhances the weld crack sensitivity. Hence, Mo is preferably added in an amount not greater than 0.60%.

V is an element which is effective in enhancing the strength of the base metal and especially effective in achieving reduction in the carbon content and carbon equivalent.

Since the addition of V in excess causes a deterioration in the toughness of the weld heat affected zone and the weld metal portion, it is preferably added in an amount not greater than 0.10%.

When added in a trace amount, B improves the hardenability of the steel and is extremely effective in providing the ultra-low-C-Nb-Ti type steel of the present invention with a high strength. However, when B is added in large amounts, B compounds precipitate at the austenite grain boundary and cause severe deterioration in the toughness of the base metal and of the weld heat-affected zone. It is, therefore, desirable that the amount of B be not greater than 0.003%.

Ce has the effects of controlling the size, and shape of sulfide-type inclusions formed in the steel, improves anisotropy, reduces the hydrogen-induced crack sensitivity, and supresses the dissolution of sulfide into the austenite matrix due to the thermal cycle of welding, hence restricting the precipitation of S at the austenite grain boundary. By way of these effects, Ce improves the toughness in the weld heat-affected zone in one-side welding with one welding pass in two-side welding with two layers. However, the addition of Ce in large amounts is not desirable because it forms sulfide-, oxide- or complex-type inclusions of Ce at the bottom of the steel ingot, and causes the occurrence of defects as apparent to an ultrasonic fault detector. Accordingly, it is recommended to add Ce in an amount not exceeding 0.02We.

In addition to the same effects as are obtained by the above-mentioned Ce, addition of fine inclusions of Ca control the coarsening of the austenite grain size in the weld heat-affected zone and prevent the formation of the martensite island by acting as the nuclei of ferrite during transformation. In order fully to achieve these effects of Ca, addition it is necessary to limit the amount of Ca to a level not greater than 0.003%, and to add Ca is preferable in amounts in the range of from 0.005 to 0.002%.

Besides the above-mentioned components, P and S are present in the steel as inevitable impurities. Although the content of these impurities is desirably as low as possible, the present invention allows the presence of up to 0.020% of P and S.

There is no particular limitation of operative conditions, e.g. in steel making or rolling, for steel in accordance with the present invention, and a production process for ordinary Nb-containing steel may likewise be employed in the present invention. Thus, it is not necessary to apply quenching and normalizing treatments to the steel after hot rolling. In other words, the steel strip as hot-rolled may be used as such without heat treatment.

Moreover, various other forms of steel strip may also be used, e.g. a steel strip produced by accelerated cooling after hot rolling, a steel strip which has been further subjected to a tempering treatment subsequent to accelerated cooling and a steel strip subjected to a quenching and tempering treatment after hot rolling. In any of the above-mentioned steel strips, it is possible to restrict the quantity of the martensite island which is formed in the weld heat-affected zone to up to 15% and to provide the steel with excellent properties such as good toughness and a good resistance to weld cracking.

The steel of the present invention is now further described with reference to the following examples.

Example 1 An 18.3 mm-thick steel strip is produced by control-rolling using each of the steel ingots having the chemical composition shown in Table 1. below. For ready reference, Table 1 also illustrates a carbon equivalent (C.E.) expressed by the formula below, and a PCM value which is generally used as a scale to express the weld crack sensitivity of the steel (the smaller the PCM value, the smaller is the sensitivity): C.E. = C+1/6Mn+1(Cr+Mo+V)+1/15(Ni+Cu) PCM = C+ 1/30Si+ 1I20(Mn+Cu+Cr) + 1/60Ni+ 1/15MO+ 1/10V+5B TABLE 1 Chemical Composition of Samples (wt %) Sample C Si Mn P S Nb A 0.03 0.11 2.17 0.012 0.005 0.053 B 0.03 0.41 1.79 0.016 0.006 0.042 C 0.03 0.18 1.80 0.015 0.006 0.056 D 0.03 0.05 1.82 0.013 0.004 0.039 E 0.02 0.45 1.43 0.014 0.006 0.057 F 0.02 0.16 1.91 0.014 0.005 0.024 G 0.04 0.28 1.38 0.015 0.006 0.062 H 0.03 0.15 2.33 0.015 0.006 0.005 I 0.02 0.15 1.77 0.017 0.003 0.029 J 0.04 0.29 2.08 0.012 0.002 0.046 K 0.06 0.11 1.87 0.014 0.005 0.053 L 0.03 0.22 1.80 0.014 0.004 0.050 M 0.06 0.36 1.73 0.017 0.005 0.042 N 0.05 0.08 1.95 0.015 0.004 0.027 0 0.11 0.09 1.51 0.012 0.004 0.022 P 0.11 0.30 1.65 0.014 0.006 0.090 Q 0.15 0.15 1.39 0.014 0.002 0.055 R 0.06 0.33 1.92 0.016 0.003 0.060 TABLE 1 (continued) Chemical Composition of Samples (wt %) Sample At Ti N Cu Ni A 0.02 0.019 0.006 - 0.27 B 0.04 0.017 0.003 0.37 0.21 C 0.06 0.023 0.005 - D 0.03 0.026 0.005 - 0.25 E 0.03 0.018 0.002 - 0.50 F 0.04 0.009 0.004 0.13 G 0.04 0.006 0.003 - H 0.03 0.028 0.002 - I 0.04 0.024 0.003 - J 0.02 0.005 0.006 - 0.88 K 0.03 0.021 0.003 - 0.28 L 0.04 0.057 0.010 - 0.35 M 0.04 0.080 0.007 - 0.51 N 0.03 - 0.003 - O 0.04 - 0.004 - P 0.03 0.019 0.005 - - Q 0.05 0.008 0.003 - R 0.03 - 0.007 - TABLE 1 (continued) Sample Cr Mo V B Ca Ce A - 0.35 - - - B ~ ~ ~ C 0.24 0.33 - - - D 0.17 0.14 - - - E - - - 0.001 0.001 F - - - 0.002 - G - - - - - - H - - 0.04 - - 0.011 I 0.36 - 0.07 - - J - 0.53 - - - K - 0.36 - - - L 0.37 - - - - M 0.16 - - - - N 0.25 0.34 - - - O - - 0.10 - - P 0.24 - - - - Q - 0.20 0.07 0.001 - R - 0.34 - - - TABLE 1 (continued) (wt Sample C+10N Ti* C.E. PCM C+ 10N A 0.09 0.21 0.48 0.17 B 0.06 0.28 0.37 0.16 C 0.08 0.29 0.44 0.16 D 0.08 0.33 0.41 0.14 E 0.04 0.45 0.29 0.11 F 0.06 0.15 0.35 0.14 G 0.07 0.09 0.27 0.12 H 0.05 0.56 0.43 0.16 I 0.05 0.48 0.40 0.14 J 0.10 0.05 0.55 0.20 K 0.09 0.23 0.46 0.19 L 0.13 0.44 0.43 0.15 M 0.13 0.62 0.41 0.18 N 0.08 - 0.49 0.19 O 0.15 - 0.38 0.20 P 0.16 0.12 0.43 0.21 Q 0.18 0.04 0.43 0.25 R 0.13 - 0.45 0.19 * no unit An impact value at the weld bond joint is examined by applying two-side submerged arc welding, with one pass in each groove, having a weld heat input of 40 KJ/cm, and one-side submerged arc welding, with one pass, having a weld heat input of 100 KJ/cm, to each of the samples A to R (thickness: 18.3 mm) shown in Table 2. (below) The "Battelle type underbead cracking test", which has the lowest heat input among circular seam welding tests, and the "Y-slit weld crack test", in accordance with JIS Z 3158, are conducted for each sample, in order to examine the resistance to weld cracking. The results are shown in Table 2 wherein samples A to J are samples of the present invention, samples K to Q are comparative samples similar to the present samples, and sample R is a comparative sample having a typical conventional Nb-containing steel composition.

TABLE 2 Test Results Sample Tensile property of Toughness at bond Crack*2 base metal (kglmm2)*1 vEo (kg-m) Ratio (%) Y.S. T.S. 50% FATT 40 KJ/cm 100 KJ/cm of base metal ( C.)*1 A 52.7 68.2 -103 12.3 11.2 4 B 49.9 60.9 -105 15.1 10.7 2 C 42.5 62.2 - 97 11.6 9.5 0 D 51.4 63.3 -101 14.1 12.2 0 E 47.1 55.1 -102 13.3 9.9 5 F 50.5 62.7 - 89 100 8.3 2 G 45.2 50.9 - 94 109 8.4 0 H 51.8 63.5 - 97 10.8 8.6 0 I 45.6 62.2 -102 11.7 9.5 1 J 71.4 79.6 - 91 9.5 8.3 3 Comparative K 51.2 68.8 - 86 5.9 3.3 8 L 44.6 63.2 - 87 4.2 2.9 5 M 52.4 61.5 - 83 6.3 3.7 7 N 48.1 66.1 - 88 4.9 3.2 9 O 53.7 65.5 - 87 2.9 2.3 37 P 54.0 67.2 - 77 2.8 1.6 32 Q 68.9 75.4 - 81 3.6 3.0 30 Prior art R 53.1 73.2 - 79 2.5 1.4 26 TABLE 2 (Continued) Crack-preventing This invention temperature "C *3 I II A 50 = 6 15 B 50 = 'r 15 C 50 = 6 15 D 25 = 6 15 E 50 = 6 15 F 50 = 6 15 G 25 = 6 15 H 50 = 'r 15 I 50 = 'r 15 J 75 0 Comparative K 125 0 L 75 0 M 125 0 N 125 0 0 175 75 P 175 75 Q 225 100 Prior art R 150 75 *1 . Direction of testpiece; o Crosswise direction *2 : Crack ratio in Battelle type underbead cracking test [using a high cellulose-type electrode; initial welding temp. = 0 C.] *3 : Root-crack preventing temperature in y-slit weld crack test.

(I) Using a high cellulose type; (II) Using low-H type electrodes: As shown in Table 2 above, the impact value (vEo) at the bond of the samples A - J of the present invention exhibits a value as high as 8 kg-m or above, irrespective of the quantity of the weld heat input. By contrast, the comparative samples K - Q though having a composition similar to that of the present samples, fail to satisfy the requirements of the C+1ON value and the Ti/(C + lON) value, and the comparative sample R has an impact value of from 2 kg-m to 6 kg-m at most. In comparison with these comparative samples, the toughness at the weld bond joint of the samples of the present invention has an excellent value which is higher by a factor of 2 to 7.

As shown in the column headed "crack ratio", whereas the resistance to weld cracking of the comparative samples K - R ranges from 5 to 40%, it is from 0% to 5% at most in the samples of the present invention, which represents a particularly advantageous feature of the invention.

As in the column headed "crack preventing temperature", the root crack preventing temperature in the Y-slit weld crack test is 125 - 175 C. for the comparative samples K - R by contrast in the samples of the present invention, this value is extremely low, i.e., generally in the range of 25 - 50"C. and 75"C. at the highest, when a high cellulose type electrode is used (column I). On the other hand, when the low hydrogen-type electrode is used (column II), the temperature is from 0 to ioooc. for the comparative samples K - R whereas it is extremely low, i.e., - 15"C. or below, in the samples of the present invention.

When compared with the values of the comparative samples K to R, these values are found to represent a further highly advantageous feature of the invention. Reference is now made to the accompanying drawings, in which: Figure I is a diagram showing the relationship between the amount of the martensite island in the weld head-affected zone and the parameter: [C(%) + 10N(%)]; Figure 2 is a diagram showing the relation between the impact value in the weld bond portion and the parameter: C(%) + 10N(%); Figure 3 is a diagram showing the relation between the impact value in the weld bond portion and the parameter: Ti (%)/[C(%) + 10N(%)]; Figures 4-[I] and - [II] are each photographs showing the microstructure near the weld bond portion (magnification:2000X); and Figure 5 is a diagram showing the influence of the weld heat input on the toughness in the weld heat-affected zone in accordance with the synthetic heat affected zone test.

Figures 1 to 3 show the results of the abovementioned Table 2 in conjunction with C(%) + 10N(%) or Ti (%)/[C(%) + 10N(%)j. In the diagrams, the symbols correspond to the sample numbers of Table 2, and the marks represent the samples in the following manner: O : Samples of the present invention : comparative samples K - Q X : comparative sample R having the conventional composition As stated above Figure 1 illustrates the relationship of the amount of the martensite island formed at the weld bond of a joint obtained by two-side submerged arc welding with one pass in each groove to the parameter [C(%) + 10N (%)]. The quantity of the martensite island formed is measured by the use of a quantitative television microscope image analyser (Q.T.M., a product of Metal Research Company). In the diagram, the full line is a curve connecting the values of the Ti-containing samples, and the dashed line is a curve connecting the values of the samples not containing Ti (samples N, 0 and R). As can be seen from this diagram, the amount of the martensite island decreases with decreasing values of the parameter; [C(%) + 10N(%)] and, when the parameter: [C(%) + 10N(%)] is restricted to values not greater than 0.10%, the quantity of the martensite island is restricted to a percentage not greater than 15%.

Figure 2 is a diagram showing the relationship of the parameter: ([C(%) + 10N (%)] to the impact value (vEo) at the weld bond of the joint obtained by two-side submerged welding with one pass in each groove. The vEo value rapidly increases as the [C(%) + 10N(%)] value decreases. From this figure 2, together with the abovementioned Figure 1, it can be appreciated that a reduction in the amount of the martensite island is an important factor in improving the toughness of the weld heat-affected zone.

Figure 3 is a diagram showing the influence of Ti/[C(%) + 10N (%)1 over the impact value (vEo) of the weld bond joint obtained by two-side submerged arc welding with one pass in each groove, the curve (I) representing the samples of the present invention and the curve (2) comparative samples.

It will be understood from Figure 3 in conjunction with the abovementioned Figure 2 that the impact value (vEo) at the weld bond of the joint can be maintained at a high value of 8-kg-m or more by ensuring that the value C (%) + 10N(%) is not greater than 0.10% and that the value: Ti (%)/[C(%) + 10N(%)] is in the range of from 0.05 to 0.60.

Figure 4-[I] and - [IIj respectively show the microscopic structure (magnification: 200x of the weld bond and the adjoining region in the use of the sample A of the present invention and of the sample R of the prior art obtained by two-side submerged arc welding with one pass in each groove. It can be seen by comparing these figures that the quantity of the martensite island formed in the bainite structure is reduced to a marked extent in the sample of the present invention (Figure 4-[I]).

Example 2 The influence of the weld heat input over the toughness of the weld heat-affected zone is examined for the sample A of the present invention and the comparative sample R, each having the composition shown in Table 1, in accordance with the synthetic heat affected zone test.

The heat cycle employed in a single heat cycle having a maximum heating temperature of 1300. C. and a cooling time in each case of 8 sec., 36 sec., 160 sec. and 250 sec. from 800"C.

to 5000C. in other words, a 2mm V-notch Charpy impact test is conducted while the steel having a thickness of 18.3mm is subjected to heat cycles each corresponding to 16 KJ/cm, 40KJ/cm, 100KJ/cm and 150 KJ/cm. The results are shown in Figure 5 in which the dashed line represents an impact value (vEo), the full line a 50% - fracture appearance transition temperature (50%-FATT;), the curve (I) represents the sample A of the present invention and the curve (2) the comparative sample R.

It can be seen from Figure 5 that, irrespective of the weld heat input quantity, the sample of the present invention has a higher impact value (vEo), a lower 50% - FATT value and better properties than the comparative sample R. In addition, degradation of 50%-FATT in the sample A is found to be smaller and less sensitive to the increase in the cooling time from 800"C. to 500"C. (increase in the weld heat input).

Example 3 By way of the Battelle type underbead test and the Y-slit weld crack test, changes in the touchness at the weld bond of the joint and resistance of weld cracking in the case of a yield strength of a class of 40 - 70 kg/mm2 are examined using steels of varying composition as shown in Table 3 (samples S and T being according to the present invention and U a comparative sample), that have been subjected to any of the following treatments: (a) rolling; (b) tempering after rolling; (c) accelerated quenching immediately after rolling; (d) tempering ater treatment (c); and (e) quenching and tempering after rolling.

The results are shown in Table 4 (below): TABLE 3 Chemical composition of samples (wt %) Sample C Si Mn P S Cu Steels of thins invention S 0.03 0.18 1.92 0.015 0.004 - T 0.02 0.13 2.14 0.012 0.003 0.15 Conventional steel U 0.06 0.10 1.86 0.013 0.006 Sample Nb Ti Ce Ca N C+10N Steels of this invention S 0.049 0.015 0.005 - 0.004 0.07 T 0.037 0.019 - 0.001 0.005 0.07 Conventional Steels U 0.056 - 0.009 - 0.005 0.11 Samples S, T and U all contain 0.02% by weight of At.

TABLE 3 (Continued) Chemical Composition of Samples (wt %) Sample Ni Cr Mo Steels of this invention S - - 0.34 T 0.81 0.15 0.43 Conventional steels U - - 0.38 Sample Ti* C.E. PCM C+1ON Steels of this invention S 0.21 0.42 0.15 T 0.27 0.56 0.19 Conventional Steels U - 0.45 0.18 *No unit Samples S, T and U all contain 0.02% by weight of At.

TABLE 4 properties of base metal* Sample Thickness Processing Tensile Test (mm) step after rolling Y.S.

(kg/mm2) S 18.3 a 49.6 S 18.3 b 55.3 S 18.3 c 57.0 S 18.3 d 59.8 S 18.3 e 63.6 T 12.7 a 72.4 T 12.7 e 78.6 U 18.3 a 51.2 U 18.3 e 65.3 *: Direction of test piece: Crosswise direction TABLE 4 (continued) Sample Properties of base metal* Tensile test Impact test DWTT* * T.S. (kg/mm2) 50% FATT( C.) 85% SATT ("C.) S 65.3 -110 -61 S 65.6 - 98 -53 S 73.8 - 95 -49 S 72.1 - 90 -46 S 73.3 - 84 -40 T 81.9 - 93 -51 T 83.5 -119 -66 U 68.8 -103 -58 U 75.2 - 88 -43 *: Direction of test piece; Crosswise direction ** Drop weight tear test. Shear area transition temperature.

TABLE 4 (continued) Sample Thickness Processing Impact property of bond (mm) step heat input 40 Kg/cm vEo (kg-m) S 18.3 a 15.5 S 18.3 b 14.2 S 18.3 c 16.7 S 18.3 d 12.1 S 18.3 e 13.9 T 12.7 a 11.8 T 12.7 e 12.2 U 18.3 a 4.3 U 18.3 e 4.7 TABLE 4 (continued) Sample Impact property of bond heat input 40 KJ/cm heat input 100 KJ/cm 50% FATT ("C.) vEo (kg-m) 50% FATT ("C.) S -18 13.3 - 5 S -23 11.8 - 7 S -12 12.1 - 4 S -20 11.3 - 0 S -16 12.5 9 T -10 13.7 - 1 T - 8 11.4 - 3 U 23 3.6 30 U 25 2.9 38 TABLE 4 (continued) Sample Weld Crack sensitivity I II (i) (ii) (iii) S 0 50 S-15 S 0 50 6-15 S 1 50 S-15 S 0 50 S-15 S 2 50 0 T 5 75 0 T 4 75 0 U 24 125 0 U 27 125 0 In the column headed "weld crack sensitivity" in Table 4 sub-headings [I] and [II] represent, respectively, the Battelle type underbead test and the y-slit weld crack test. The sub-heading (i) refers to an underbead cracking ratio (%) at the weld initial temperature of 0 C.; (ii) to a root crack preventing temperature ("C.) when a high cellulose type electrode is used and (iii) to a root crack preventing temperature ("C.) when a low hydrogen-type electrode is used.

As can be seen from Table 4, the properties in the base metal and the weld bond of the joint of the present samples S and T of the invention are better than those of the comparative sample U. In particular the advantages of the present invention in the properties of the welded bond and in the resistance to weld cracking remain unaltered when steel is subjected to various heat treatments after hot rolling. Thus, the present steels are found to maintain excellent weldability.

WHAT WE CLAIM IS: 1. A niobium-containing weldable structural steel having good weldability, which consists of: 0.005 - 0.04% of C; 0.01 - 0.50% of Si; 1.20 - 2.50% of Mn; 0.01 - 0.07% of Nb; 0.005 - 0.030% of Ti; 0.005-0.06% of At; N in amounts satisfying the relationship: [C(%) + 10N(%)] is not greater than 0.10% and Ti (%)/ [C(%) + 10Nt%)] is from 0.05 to 0.60, the balance being iron and inevitable impurities whereby the amount of the martensite island in the weld heat-affected zone is restricted to not more than 15% in terms of the area fraction.

2. A niobium-containing weldable structural steel having good weldability which consists of 0.005 - 0.04% of C; 0.01 - 0.50% of Si, 1.20 - 2.50% of Mn, 0.01-0.07% of Nb, 0.005 - 0.030% of Ti, 0.005 - 0.06% of At, at least one element in the specified amount selected from the group consisting of up to 0.50% Cu, up to 1.50% Ni, up to 0.50% Cr, up to 0.60% Mo, up to 0.10% V, up to 0.003% B, up to 0.02% of Ce and up to 0.003% of Ca, and N in amounts satisfying the relationship: [C(%) + 10N (%)] is not greater than 0.10% and Ti (%)/[C(%) + 1N(%)] is from 0.05 to 0.60%, the balance being iron and inevitable impurities whereby the amount of the martensite island in the weld heat-affected zone is restricted to not more than 15% in terms of the area fraction.

3. A niobium-containing weldable structural steel as claimed in claim 1, substantially as herein described with reference to the accompanying drawings and/or any of the specific examples.

4. Pipes or tubes composed of a niobium-containing weldable structural steel as defined

Claims (1)

1. in claim 1.

   5. Pipes or tubes composed of a niobium containing weldable structural steel as defined in claim 2.

   6. Pipes or tubes composed of a niobium-containing structural steel as defined in claim 3.