DE2809795A1 - Niobium-contg. constructional steel - with improved weldability, used for oil and gas pipelines - Google Patents

Abstract

The constructional steel has the compsn. (by wt.) 0.005-0.04% C, 0.01-0.50% Si, 1.20-2.50% Mn, 0.01-0.07% Nb, 0.005-0.030% Ti, 0.005-0.06% Al, balance Fe, the (C% + 10N%) content being is not >0.10% and the Ti%/(C% + 10N%) content being 0.05-0.60. The content of martensite islands in the heat affected zone is limited to is not >15% of an area section. The steel may also contain one or more of =0.50% Cu, =1.50% Ni, =0.50% Cr, =0.60% Mo, =0.10% V, =0.003% B, =0.02% Ce and =0.003% Ca. The steel is useful for oil and natural gas pipelines use in cold regions. It has a yield strength of 40-70 kp/mm2, good heat-affected zone toughness and improved weld cracking resistance even when using high heat input welding techniques.

Description

description

The present invention relates to a niobium-containing weldable Structural steel with improved weldability, consisting of o, oo5 to o, o4% C, o, ol to 0.50% Si, 1.20 to 2.50% Mn, o, o1 to o, o7% Nb, o, oo5 to 0.030% Ti, o, oo5 up to 0.06% Al, the remainder iron and unavoidable impurities, with +10 N (%)] not greater than 0.10% and Ti (%) / [C (%) + 1o N (%)] is between 0.05 and 0.60 by the Percentage of martensite islands in the welding heat-affected zone no longer than 15% of the area share.

The invention thus relates to a niobium-containing weldable Structural steel with improved weldability. In particular, this relates to Invention on a niobium-containing weldable structural steel with a yield strength from 40 to 70 kgf / mm² made by using a niobium-containing steel in a very specific composition was used to thereby increase the toughness in the weld heat affected zone and to improve weld crack resistance.

Niobium improves the strength and toughness of steel when it does Steel is added in a small amount and is also an economically advantageous one Element. For this reason, a niobium-containing steel (hereinafter referred to as "Nb-containing Steel ") widely used as weldable structural steel for pipelines, for shipbuilding, for pressure containers, bridges or the like. Found.

Among the many possible uses for structural steel containing Nb only the one for line pipe steels that is used in particularly large quantities is intended here will be described in detail. Nb-containing, not hardened, tempered Steel with high tensile strength has so far been used in large quantities for pipelines for transportation been used by crude oil and natural gas.

However, these pipelines that have been laid so far only have one small pipe diameter and a low internal pressure and are at a relatively high temperature of not less than about oOC used. That's why they were Requirements for toughness in the weld heat affected zone of not too great significance.

Since the laying of pipelines in ultra-cold regions such as Russia, Canada, USA etc. has concentrated and the material transported has increased shifted from crude oil to natural gas, an unexpected problem suddenly occurred the foreground, which consisted in the fact that the conventional Nb-containing steel for Pipelines have a brittleness of the weld heat-influenced zone (HZ) in the weld seam area caused at the time of steel making and therefore insufficient toughness may have to understand the application in the aforementioned territories. Countermeasures are, on the one hand, a multiple electrode welding process with a concealed Arc and with low welding heat input or the so-called MIG welding process examined from the point of view of reducing the influence of welding heat and on the other hand are studies and developments on the field of steel material used has been made to make a steel with To obtain such a composition of economic components, of none Deterioration in the weld heat affected zone causes more.

Especially with regard to the properties of the steel material the steel that did not affect embrittlement by the heat of welding would be the most desirable Zone causes more and has good impact strength, even if that Welding with high welding heat input, such as with one-sided welding or when welding on both sides with one or two layers (but not more than three plots in each welding groove) is carried out to improve welding efficiency to improve and reduce the manufacturing cost of the pipe. At the same time must the steel meets the specific requirement of low sensitivity to weld cracking genes to prevent cracks in the weld area because of a construction site weld in the cold area mentioned above, basically a cellulose type electrode with a high hydrogen content is used, whereby a preheating of the pipe is not necessary in this cold environment.

For those skilled in the art concerned with these problems, it therefore arose the urgent need to develop a steel strip for a conduit pipe, which at the same time satisfies the above-mentioned requirements, i.e., excellent Has toughness in the weld heat-affected zone, is poorly hardenable and excellent weld crack resistance; having.

The following US patents refer to the state of the art: 3 592 633, 3 807 99o, 3 619 303, 3 725 o49, 3 721 587 and 3 132 o25.

The object of the invention is now to solve the problems identified above, which arise when welding a steel containing Nb, in a rational manner solve and a Nb-containing weldable structural steel, in particular a Nb-containing Find structural steel for pipelines that has good toughness in the heat of welding Zone and has improved weld crack resistance.

To solve this problem, a first embodiment of the present Invention of an Nb-containing weldable structural steel with improved weldability before, consisting of o, oo5 to o, o4% C, o, o1 to 0.50% Si, 1.20 to 2.50% Mn, o, o1 up to o, o7% Nb, o, oo5 to o, 30% Ti, 0.005 to o, o6% Al, the remainder iron and unavoidable Impurities, where [C (%) + 1o N (% l7 is not greater than 0.10% and Ti (%) / + 10 N (%)] is between 0.05 and 0.60, the proportion of martensite islands in the Welding heat-affected zone to not more than 15% of the area cross-section restrict.

A second aspect of the present invention provides one Nb-containing weldable structural steel with improved weldability from 0.005 to 0.04% C, 0.01 to 0.50% Si, 1.20 to 2.50% Mn, 0.01 to 0.07% Nb, o, oo5 to o, o30% Ti, o, oo5 to o, o6% Al, at least one element in the prescribed amount, selected from the group consisting of up to 0.50% Cu, up to 1.5c% Ni, up to 0.50% Cr, up to 0.60% Mo, up to 0.1o% V, up to 0.03% B, up to 0.02% Ce and up to 0.03% Ca, the remainder being iron and unavoidable Impurities, where / C (%) + 10 N (%) / is not greater than 0.1o% and Ti (%) / [C (%) + 10 N <%) 7 lies between 0.05 and 0.60%, around the proportion of martensite islands in the welding heat-affected zone to not more than 15% of the area cross-section to restrict.

The third embodiment provides for the first embodiment of the invention the Nb-containing weldable structural steel for line pipes.

The fourth embodiment provides for the second embodiment of the invention the Nb-containing weldable structural steel for line pipes.

The invention will now be based on exemplary embodiments in conjunction will be explained in more detail with the accompanying drawings. They show: FIG. 1 in one Diagram showing the relationship between the amount of martensite islands in the weld heat affected Zone and the expression L C (%) + 10 N (% t7; Fig. 2 in a diagram the relationship between the weld joint area notch toughness and the term C (%) + 1o N (%); 3 is a graph showing the relationship between the notch toughness in the weld joint area and the expression Ti (%) / [C (%) + 10 N (% l7; Fig. 4a and 4b each in a photographic representation of the microstructure near the weld joint area (Magnification: 2oox); and FIG. 5 shows in a diagram the influence of the introduced Welding heat on the toughness of the welding heat-influenced zone in experiments with a zone affected by artificial heat.

It is known per se that the welding heat-influenced zone, in particular the weld joint itself and the areas near that weld joint are embrittled if any weldable structural steel with high welding heat input, such as in automatic welding, welding is carried out in order to reduce this embrittlement Avoid, will see it as an effective angel, the structure of the welding heat-affected Zone in fine ferrite pearlite, lower bainite or in a mixed structure of lower Converting bainite and martensite, however, this process cannot be effective on a steel containing Nb, with which the present invention concerned, applied; because the zone formed in the welding heat-affected zone Structure varies depending on the hardenability of the steel. For example, if the ferrite-pearlite structure is desired, the hardenability does not have to be so high, so that thereby the proportions of the alloying elements to be added inevitable be restricted. Therefore, this method is only applicable to a steel which consists essentially of a Si-Mn system with a yield strength in the order of magnitude of 20 to 40 kp / mm2, and which has only low alloy additions. When this method is applied to a steel containing Nb, an upper Obtained bainite structure, which has extremely low toughness and on the contrary promotes embrittlement of the heat affected zone.

On the other hand, a low bainite structure or a mixed structure To get from low bainite and martensite, it is necessary to use a steel too which contains expensive elements such as Ni, Cr, Mo, etc. in large quantities. However, such a steel containing large amounts of these elements has a extremely low weld crack resistance and causes a drastic increase the manufacturing costs. For these reasons, it is difficult to make a steel this Type to be used as weldable structural steel.

In the steel containing Nb, the weld seam of the The connection and the areas near the weld seam can be quickly reduced to approx. 13000C and more heated.

This removes the Nb carbonitrides that are formed during the rolling of the product or deposited after hardening and tempering, by the heat of welding thermally decomposed and dissolved in the matrix, so that it is essentially a significant Increase in hardenability, hardening of the weld seam and its surroundings and cause a reduction in weld cracking resistance. The hardening of the welding heat affected Zone can therefore theoretically go back into solution by reducing the Amount of Nb can be prevented.

However, this diminution is in contrast to the essence of the Nb-containing steel.

Extensive experiments were carried out in the context of the present invention Taken with the aim of finding out the reasons for the deterioration in toughness in the Find out welding heat-affected zone and countermeasures and a procedure to improve the weld cracking resistance of a steel containing Nb, The most essential feature of this Nb-containing steel is said to be that it has high strength at low cost, making this steel as weldable Structural steel, particularly suitable as steel for line pipes.

In the course of these investigations it has now been clarified that the lump-like or spherical structures, called "martensite islands", found in the upper Bainite structure occur and in the weld heat-affected zone at the time of Welding with high welding heat input, for example when welding on one side with one deposit or when welding on both sides Welds are formed with a weld deposit in each weld groove, is the starting point or transmission path of the embrittlement cracking and that, if the amount of martensite islands exceeds 15%, the toughness of the steel is extraordinary is worsened. Therefore, investigations were continued on a steel composition to obtain the negative effects resulting from the formation of the martensite islands Influence is reduced and the weld cracking resistance is improved, it being found that that the proportions of martensite islands can be limited to less than 15% and that the weldability is achieved by determining the proportions of C, Si, Mn, Nb, Ti etc. to a specific ratio as well as by setting the proportions of C, Ti and N can be remarkably improved to a certain relationship with each other can. The present invention is completed on the basis of this finding.

Particularly in accordance with the present invention A weldable structural steel containing Nb is planned, consisting of o, oo5 to o, o4% C, o, o1 to 0.50 8 Si, 1.20 to 2.50% Mn, o, o1 to 0.07% Nb, 0.005 to o, o3o Ti, o, oo5 to o, o6% Al, at least one element in the prescribed amount, if necessary, and selected from a group consisting of up to 0.50% Cu, up to to 1.0% Ni, up to 0.50% Cr, up to OD6O% Mo, up to 0.10% V, up to 0.03% B, up to 0.02% Ce and up to 0.003% Ca, the remainder iron and unavoidable Impurities, where L C (%) + 10 N (%) 7 is not greater than 0.1o% and Ti (%) / [C (%) + 10 N (%)] between o, o5 and 0.60 to the proportion of martensite islands in the welding heat-affected Limit zone to no more than 15% and have a negative impact on toughness and to effectively prevent or mitigate the susceptibility to weld cracking.

With the present invention it is possible to achieve the above Solve problems that occur when welding Nb-containing steel as a whole, the weld heat affected zone with high toughness and high weld crack resistance during welding with high welding heat, for example when automatic or semi-automatic welding, including one-sided welding with a weld deposit, double-sided welding with two layers and the To provide circumferential welding and a yield strength in the order of magnitude of to guarantee approx. 40 to 70 kp / mm² The following should now explain why the components of the Nb-containing steel of the present invention to a specific one Ratio are limited.

In cooperation with Ti and Na which will be described later, A reduced C content ensures that the formation of martensite islands is restricted is that the toughness in the 5chwelßhitze affected zone is favored and d & b is the containing steel of the present invention special Can bring properties to full advantage.

Therefore, the amount of C added must be given particular careful attention be given. To improve toughness and resistance to welding cracks by reducing the amount of formation of martensite islands that affected in the welding heat Zone occur, the C content must be reduced and must not be greater than o, o4%. In this way it is possible, by restricting the C content in Relationship with the Ti and N proportions, which will be described later, the proportion at martensite islands to not more than 15%. Although the carbon content is preferably kept as small as possible, a particular problem arises the cost of production if it is less than o, oo5 in a practical steel % should be reduced.

Therefore, for practical reasons, the carbon content is in the range from 0.005 to 0.04%.

Also, it is necessary to be related to the carbon content Ti or N to restrict. When the carbon content is not greater than the above described upper limit of 0.04% or in the practical range of 0.005 to o, o4%, it is difficult to determine the undesirable influence of the martensite islands on to completely eliminate the toughness in the weld heat affected zone. That's why In addition to these restrictions, the C content in the compound with N, i. the relationship [[C (%) + 1o N (% i7) must be limited to not greater than 0.10%, and in conjunction with Ti and N, the relationship Ti (%) / LC (%) + 1o N (%)] must be in the range from o, o5 to 0.60. These restrictions impose on the heat affected Zone has excellent notched impact strength.

Ti has the effect of increasing the formation amount of martensite islands reduced in the welding heat-affected zone and also the toughness in this Zone promotes, at the same time increasing the austenite grain size in the heat-affected zone, in particular in the weld seam and in the area close to it the weld seam, prevented. When the Ti content is less than 0.05%, will the effect of the TiN in preventing the austenite grain size from increasing in size insufficient; it also becomes difficult to fix the harmful free nitrogen or render it harmless. Therefore, at least 0.05 t Ti should preferably be added be.

On the other hand, the addition of an excessive amount of Ti is not desirable because it increases the proportion of Tifl in the steel or the formation caused by large Ti-type inclusions and not only affected toughness in the welding heat Zone but also deteriorated in the base material.

Therefore, the upper limit of the Ti content is preferably 0.030 %.

It is also an essential requirement in the present one Invention that, as mentioned above, the Ti content in connection with C and N is limited in such a way that the value Ti (%) / L C (%) + 10 N (%)] is in the range of 0.050 to 0.60 with the proviso that C (%) + lo N (%) is 0.10%. This represents one of the features of the present Invention that the proportion of the martensite islands that occur in the weld heat affected zone more than 15% is restricted. When the value Ti (%) / L C (%) + 10 N (%)] is less than Is 0.05, it is difficult to make the martensite island fraction satisfactory reduce, and on the other hand, when the value is greater than 0.6o, the above occurs mentioned the inverse influence of Ti and reduced the toughness of the heat-affected Zone. For these reasons, the Ti content must be in the range from 0.05 to 0.030% and at the same time the value Ti / C (%) + 1o N (iL7 are in the range from o, oS to 0.60, where the value C (%) + 10 N (%) to not more than 0.1o%, as mentioned above, is limited.

Nb is the basic element for the steel of the present invention, which is referred to as so-called Nb-containing steel.

Nb is extremely effective for improving strength and toughness of steel and also an economic element. The effect of the added Although Nb increases with an increasing proportion of the addition to the base metal, it does tend to both toughness in the weld heat affected zone and weld crack resistance to gradually deteriorate. In addition, the addition of a large proportion of Nb not economical. For all these reasons, the upper limit of the proportion is set preferably at o, o7%, while the lower limit is preferably o, ol%, because the properties of the Nb-containing steel are not achieved satisfactorily can if the additional amount is too small.

As mentioned earlier, N is an element that has a considerable effect affects the toughness in the weld heat affected zone in the same way as C. The range of the N content is determined in connection with the C and Ti contents. In particular, the N content must meet the following two requirements: C (z) + lo N (%) <= 0.10 g (1) o, oo5 = Ti (%) / LC (%) + 1o N (%)] - 0.60 (2) if for example the carbon content at the practically meaningful lower limit of o, oo5% and the Ti content is at the upper limit of 0.03%, the N content must be less than o, oo95% (pol) from formula (1) and at o, oo45 to o, o595% (ß) from the formula (2) lie. Thus, the N content must be between 0.045 and 0.095%, at the same time <OL) and (ß) are fulfilled. On the other hand, when the C content is at its upper limit of 0.04% and the Ti content is at its lower limit of 0.05% the N content is not greater than 0.06% () from formula (1) and not greater than 0.06 % (&) from formula (2). In this case, the N content is no more set as o, oo6%, so that both (γ)) and () are fulfilled at the same time are.

Si ensures the strength of the base metal and acts as a deoxidizer in steel production. For these reasons, 0.1 to 0.50% Si is added.

In the same way as the above-mentioned Si, Mn is added, to give the steel the required strength. If the proportion is below 1.2%, it is difficult to get a yield strength on the order of 40 kp / mm2 in the steel according to the invention with the extremely low carbon content obtain. Therefore, at least 1.2% of Mn is preferably added. If Mn im If excess is added, the Mn deposition in the raw steel ingot is promoted, whereby not only deteriorates the purity of the steel, but also the formation of martensite islands in the weld heat-affected zone, which favors hardenability and the toughness and the pig crack resistance are lowered. For this reason the upper limit of the Mn content is preferably not greater than 2.5%.

Al is used as a deoxidizing element during steel production and also effective as a grain-refining element.

But it also acts as a nitride-forming element and binds the free Nitrogen in the weld heat-affected zone, as well as it for stabilization and improving toughness in the weld heat affected zone is effective.

However, addition of Al in excess is not desirable because it causes an increase in alumina-like inclusions and the purity of steel diminished. Preferably Al is used in proportions ranging from 0.005 to o, o6% added.

In addition to the elements described above, the Nb-containing Steel in accordance with the present invention also still if so is required, mixed crystal elements, such as Cu, Ni, Cr and Mo, and trace elements, such as V, B, Ca and Ce contained in appropriate amounts to thereby improve the toughness and other various properties such as further improving strength.

However, it does not need to be mentioned separately that these additional elements must be added within such content limits that the toughness in the weld heat affected zone and the weld crack resistance is not deteriorated will. There is also a certain limit on the amount of each to be added Element with regard to the particular influence of the element on the properties of the steel strip, such as the strength and toughness, and also from the aspect of Manufacturing technique imposed from.

Cu increases the strength without causing any negative effect the toughness of the base metal and the weld heat affected zone and improves hydrogen-induced crack resistance and corrosion resistance.

However, the upper limit is set at 0.5o% because one Exceeding this limit cracks on the surface of the steel belt during the Rolling occur.

Although Ni a remarkable improvement in the toughness of the base metal and the weld heat affected zone, the addition of Ni is in excess for a welded Construction not desirable because it means the serious problem of stress corrosion cracking occurs and this leads to an increase of production costs. It is therefore desirable that Ni be in an amount of not greater than 1.50% is added.

Cr is a useful element for ensuring the strength of the base metal. However, adding Cr in an excessive amount causes hardening of the weld heat affected Zone and reduces the weld crack resistance.

For this reason, Cr is preferably used in an amount of not more added as o, 5o%.

Mo is also a useful element in maintaining the Strength of the base metal. However, if it is added in an excess amount is, this Mo causes the martensite island proportions to increase, is lowered the toughness of the weld heat-affected zone and favors the susceptibility to weld cracks. Therefore, Mo is preferably added in an amount not larger than 0.6o%.

V is an element that has an effective influence on strength of the base metal has and is particularly effective in achieving a reduction the carbon content and the carbon equivalent. Since adding V in the excess deterioration in toughness in the weld heat affected zone and the weld metal area entails, it is preferably in a tight spot of no more than o, 1o% added.

When added in traces, B improves the hardenability of steel and is extremely effective for creating an ultra-low C-Nb-Ti type of steel according to of the present invention with high strength. However, if B in a large amount is added, the B components precipitate at the austenite grain boundaries and deteriorate the toughness of the base metal and the weld heat affected zone are extraordinary. It is therefore desirable that the proportion of B is not more than 0.03%.

Ce acts to control the size and shape of those formed in the steel Sulphide-type inclusions, improves the anisotropy, reduces the hydrogen-induced Crack sensitivity, suppresses the dissolution of the sulfide in the austenite matrix as a result of the thermal welding cycles and thus suppresses the precipitation of S on the Austenite grain boundaries. With these effects, the Ce improves the toughness in the Welding heat-affected zone for one-sided welding with one welding sequence or when welding on both sides with two welding layers. However, this must be added of Ce in a large amount is undesirable because it is sulfide, oxide, or complex-like Inclusions of Ce in the bottom area of the raw steel block are formed and the occurrence of Causes defects when using an ultrasonic flaw indicator measuring device. the end for this reason, Ce should be added in an amount not larger than 0.02%.

In addition to the same effect as the above-mentioned Ce fine inclusions of Ca control the enlargement of the austenite grain size in the Heat-affected zone and prevent the formation of martensite islands, there they serve as cores for the ferrite during conversion. To these effects of the To completely exclude Ca, it is necessary to limit the amount of Ca to no more than o, oo3 % and preferably add it in a range of 0.05 to 0.002%.

Besides these components mentioned above, P and S are in steel contained as unavoidable impurities.

Although the content of these impurities is preferably so low should be as possible, the present invention allows up to to o, o20 z to P and S.

There are no particular restrictions on the operating conditions in steel making, rolling etc. of the steel according to the present invention, so that the manufacturing process for the ordinary Nb-containing steel, for example can be applied to the present invention. It is not necessary for apply hardening and normalizing treatments to the steel after hot rolling, i.e.

the hot-rolled steel strip can be used as it is without heat treatment be used. Various other steels can also be used such as a steel that is subjected to accelerated cooling after hot rolling has been produced, a steel in which then to the protruding The accelerated cooling mentioned above has also been subjected to a tempering treatment and a steel strip which is subjected to hardening and tempering treatment after hot rolling has been subjected. With each of the steels mentioned above it is possible the amount of martensite islands formed in the weld heat affected zone down to 15% and the steel with excellent properties, such as e.g. good toughness and weld crack resistance.

The steel according to the invention will now be described in detail on the basis of the following Examples are explained in more detail.

Example 1 A 18.3 mm thick steel is controlled by rolling using one raw steel block with the chemical shown in Table 1 Composition has been made.

For the sake of clarity, Table 1 also shows the carbon equivalent (C.E.) which is expressed by the following formula and a PCM value which is basically used as a yardstick for the sensitivity of steel to weld cracks (a smaller PCM value means a smaller sensitivity).

C.E. = C + 1/6 Mn + 1 (Cr + Mo + V) + 1/15 (Ni + Cu) PCM = C + 1/30 Si + 1/20 (Mn + Cu + Cr) + 1/60 Ni + 1/15 Mo + lilo V + 5B Tabel 1 Chemical composition of the samples (% by weight) Sample C Si Mn P S Nb Al Ti N Cu Ni A 0.03 0.11 2.17 0.012 0.005 0.053 0.02 0.019 0.006 - 0.27 B 0.03 0.41 1.79 0.016 0.006 0.042 0.04 0.017 0.003 0.37 0.21 C 0.03 0.18 1.80 0.015 0.006 0.056 0.06 0.023 0.005 - -D 0.03 0.05 1.82 0.013 0.004 0.039 0.03 0.026 0.005 - 0.25 E 0.02 0.45 1.43 0.014 0.006 0.057 0.03 0.018 0.002 0.13 -F 0.02 0.16 1.91 0.014 0.005 0.024 0.04 0.009 0.004 0.13 -G 0.04 0.28 1.38 0.015 0.006 0.062 0.04 0.006 0.003 - -H 0.03 0.15 2.33 0.015 0.006 0.055 0.03 0.028 0.002 - -I 0.02 0.15 1.77 0.017 0.003 0.029 0.04 0.024 0.003 - -J 0.04 0.29 2.08 0.012 0.002 0.046 0.02 0.005 0.006 - 0.88 K 0.06 0.11 1.87 0.014 0.005 0.053 0.03 0.021 0.003 - 0.28 L 0.03 0.22 1.80 0.014 0.004 0.050 0.04 0.057 0.010 - 0.35 M 0.06 0.36 1.73 0.017 0.005 0.042 0.04 0.080 0.007 - 0.51 N 0.05 0.08 1.95 0.015 0.004 0.027 0.03 - 0.003 - -O 0.05 0.08 1.95 0.015 0.004 0.027 0.03 - 0.003 - -P 0.11 0.30 1.65 0.014 0.006 0.090 0.03 0.019 0.005 - -Q 0.15 0.15 1.39 0.014 0.002 0.055 0.05 0.008 0.003 - -R 0.06 0.33 1.92 0.016 0.003 0.060 0.03 - 0.007 - - Table 1 (continued)% by weight Sample Cr Mo V B Ca Ce C + 10N Ti * C.E. PCM C + 10N A - 0.35 - - - - 0.09 0.21 0.48 0.17 B - - - - - - 0.06 0.28 0.37 0.16 C 0.24 0.33 - - - - 0.08 0.29 0.44 0.16 D 0.17 0.14 - - - - 0.08 0.33 0.41 0.14 E - - - 0.001 0.001 - 0.04 0.45 0.29 0.11 F - - - 0.002 - - 0.06 0.15 0.35 0.14 G - - - - - - 0.07 0.09 0.27 0.12 H - - 0.04 - - 0.011 0.05 0.56 0.43 0.16 I 0.36 - 0.07 - - - 0.05 0.48 0.40 0.14 J - 0.53 - - - - 0.10 0.05 0.55 0.20 K - 0.36 - - - - 0.09 0.23 0.46 0.19 L 0.37 - - - - - 0.13 0.44 0.43 0.15 M 0.16 - - - - - 0.13 0.62 0.41 0.18 N 0.25 0.34 - - - - 0.08 - 0.49 0.19 O - - 0.10 - - - 0.15 - 0.38 0.20 P 0.24 - - - - - 0.16 0.12 0.43 0.21 Q - 0.20 0.07 0.001 - - 0.18 0.04 0.43 0.25 R - 0.34 - - - - 0.13 - 0.45 0.19 * none Unit of measurement The notch toughness in the weld of the joint was tested by using a double-sided welding with a hidden arc with one application in each welding groove when welding heat is applied of 40 KJ / cm and by using one-sided welding with a hidden arc with an application with a welding heat input of 100 KJ / cm for each of the Samples A to R (thickness: 18.3 mm) shown in Table 2.

The Battelle type underbead cracking test), which has the lowest heat input among the circumferential weld seam processes and the "Y-slot weld rupture test" in accordance with JIS Z 3158 were run on each sample to check weld crack resistance. The results are shown in Table 2, where A to J are the samples according to the present invention Invention, K to Q comparative examples which are similar to the samples according to the invention and R is a comparative example with a typical conventional Nb-containing one Represent steel composition.

Table 2: Test results of strength properties - 50% FATT toughness of the breakage - breakage preventing sample of the base metal of the weld seam behaves - temperature (° C) (kg / mm²) +1 basic me- vEo (kg-m) nis # (%) # 3 stretch - Tensile strength strength (° C) # 1 40 KJ / cm 100 KJ / cm I II Invention A 52.7 68.2 -103 12.3 11.2 4 50 # -15 B 49.9 60.9 -105 15.1 10 .7 2 50 # -15 C 42.5 62.2 - 97 11.6 9.5 0 50 # -15 D 51.4 63.3 -101 14.1 12.2 0 25 # -15 E 47, 1 55.1 -102 13.3 9.9 5 50 # -15 F 50.5 62.7 - 89 10.0 8.3 2 50 # -15 G 45.2 50.9 - 94 10.9 8 .4 0 25 # -15 H 51.8 63.5 - 97 10.8 8.6 0 50 # -15 I 45.6 62.2 -102 11.7 9.5 1 50 # -15 J 71, 4 79.6 - 91 9.5 8.3 3 75 0 Comparative examples K 51.2 68.8 - 86 5.9 3.3 8 125 0 L 44.6 63.2 - 87 4.2 2.9 5 75 0 M 52.4 61.5 - 83 6.3 3.7 7 125 0 N 48.1 66.1 - 88 4.9 3.2 9 125 0 O 53.7 65.5 - 87 2.9 2.3 37 175 75 P 54.0 67.2 - 77 2.8 1.6 32 175 75 Q 68.9 75.4 - 81 3.6 3.0 30 225 100 State of the art R 53.1 73 , 2 - 79 2.5 1.4 26 150 75 1: direction of the test piece; cross direction * 2: fracture ratios in the Battelle-type weld seam breakage test (use of a cellulose-type electrode with a high cellulose content; initial welding temperature = 0 ° C) * 3: root breakage-preventing temperature in the Y-slot weld seam breakage test (I) use of a Cellulose-type high cellulose electrode; (II) Using a low hydrogen type electrode.

As shown in Table 2 above, shows the notch toughness (vEo) in the seam of samples A to J of the present invention has a value of approx. 8 kg-m or more regardless of the amount of welding heat input. In contrast in addition, the comparative samples K to Q, which, although they are one of the invention Samples have similar composition, the requirements of the C + 1o N value and the Ti / (C + 10 N) value do not meet, and the comparative sample R is listed, The latter has a notch toughness of 2 kg-m to 6 kg-m at the most. In comparison with these comparative samples, the toughness in the weld seam the joining of samples according to the present invention is excellent Value, which is about 2 to 7 times higher.

As shown in the column "Breakage Ratio", the weld cracking resistance lies for the comparative samples K to R between 5 and about 40% while it in the samples according to the present invention between 0% and a maximum of 5% and is therefore extremely excellent Temperature is shown, the root breakage preventing temperature is at the Y-slot Weld seam rupture test at approx. 125 to 1750C for the comparative samples K to R, while it is extremely low for the samples of the present invention, i.e. about 25 to 50 ° C and a maximum of 75 ° C if a cellulose-type electrode with high Cellulose content is used (column 1). On the other hand, when a low hydrogen type electrode is used (column II), the temperature is between 0 and 100 oC for the comparison samples K to R, while they are extremely low in the samples of the present invention i.e. at -150C and below. Compared to the values of the comparison samples K to R, these values found are extremely good.

The accompanying drawings 1 to 3 are diagrams of which each the results of Table 2 above as a function of the value C (%) + 10 N (%) or Ti (%) / + 10 N (%)]. The symbols correspond in the diagrams the sample numbers in Table 2, with the characters used following sample numbers are assigned: 0 samples according to the present invention to comparison samples K to Q X: comparative sample R with a conventional steel composition.

Fig. 1 shows the relationship between the proportions in the weld the connection formed martensite islands, whereby the weld seam through double-sided Welding with a hidden arc, each with a weld deposit in of each welding groove and the condition / C (%) + lo N (% 17 was met. The amount of martensite islands formed was determined by using a TV microscope image amount analyzer (Q.T.M), a product of Ketal Research Company. In the diagram shows the solid line is a curve showing the values of the Ti-containing samples connects, while the dashed line shows a curve showing the values of samples without Ti (samples N, 0 and R) connects.

As can be seen from this diagram, the proportions decrease of martensite islands with decreasing value of / C (%) + 10 N (%) 7 and when L-C (8) + 1c N (%) 7 is limited to not more than 0.1o%, is the amount of martensite islands limited to no more than about 15%.

Fig. 2 shows a diagram in which the relationship between L C (B) + 10N (%)] and the notch toughness (vEo) in the weld seam of the connection, which through a double-sided welding with a concealed arc with a weld seam build-up was obtained in each welding groove. The vEo value increases steeply when the L C (8) + 10 N (%)] value decreases. From this Fig. 2 together with that explained above It can be seen from FIG. 1 that a reduction in the proportions of martensite islands as an important factor for the improvement of the toughness in the welding heat-affected Zone works.

The diagram in Fig. 3 shows the influence of Ti // C (%) + 10 N (%)] on the notch toughness (vEo) of the weld seam of the connection, which is caused by double-sided Welding with a concealed arc with one weld seam in each case Groove was obtained. In this diagram, curve (1) shows the samples after present invention, while curve (2) shows the comparative samples.

From this FIG. 3 in conjunction with the above-explained FIG. 2 it can be seen that the notch toughness (vEo) in the weld seam of the connection can be maintained at a high value of 8 kg-m or more if the value [C (%) + 10 N (% L7 to not more than 0.10% and the value + 10 N (%)] in the range of 0.05 is held to 0.60.

FIGS. 4a and 4b show the microstructures in a magnification of 200 times the weld and its surroundings for Sample A of the present invention and for the sample R according to the prior art, the welds each going through Double-sided welding with a concealed arc, each with one welding pass was carried out in each welding groove.

By comparing these figures it can be seen that the amount of the martensite islands formed in the bainite structure to a considerable extent in the sample according to the invention is reduced (Fig. 4a).

Example 2 The influence of the welding heat introduced on the toughness the welding heat-affected zone was for the inventive sample A and the Comparative sample R, the samples each having the composition shown in Table 1 tested in tests with zone affected by artificial heat.

The heat cycle applied consisted of a single heat cycle with a maximum heating temperature of 13000C and a cooling time of 8 sec, 36 sec, 160 sec and 250 sec from 800C to 500 ° C. In other words, it became a Charpy impact test performed on -2 mm V-notches during the heat cycles, respectively 16 KJ / cm, 40 KJ / cm, 100 Ks / sm and 150 KJ / cm on a steel with a thickness of 18.3 mm were used.

The results are shown in Fig. 5, in which the dashed line the notch toughness (vEo), the solid line the 50% fracture transition temperature (50% -FATT; vTrs), where curve (1) the inventive sample A and the curve (2) represents the comparative sample R.

From Fig. 5 it can be seen that regardless of the amount of introduced welding heat, the sample according to the invention has a higher notch toughness (vEo), has a lower 50% FATT and better properties than the comparison sample R.

It was also found that the decrease in the 50% -FATT value in the Sample A is less and less from increasing the Cooling time at the cooling from 8000C to 5oo0C depends (increase in welding heat input).

Example 3 Using the Battelle-type weld bead base> break test and the Y-slot weld break test, the changes in the toughness of the weld seam of the joint and in the weld crack resistance in the case of a tensile strength of the order of 40 to 70 kgf / mm² were tested, with steels of varying composition as shown in Table 3 (Samples S and T as inventive samples and V as comparative sample), each of which was subjected to the following treatment steps: (a) rolling (b) tempering after rolling (c) accelerated Cooling immediately after rolling (d) Tempering after treatment (c) (e) Quenching and tempering after rolling The results are shown in Table 4. Table 3 Chemical Composition of Samples (wt%) Sample C Si Mn PS Cu Ni Cr Mo Steels according to the 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 0.81 0.15 0.43 Conventional steels U 0.06 0.10 1.86 0.013 0.006 - - - 0.38 Sample Nb Ti Ce Ca N C + 10N Ti * CE PCM C + 10N Steels according to the invention S 0.049 0.015 0.005 - 0.004 0.007 0.21 0.42 0.15 T 0.037 0.019 - 0.001 0.005 0.07 0.27 0.56 0 , 19 Conventional steels U 0.056 - 0.009 - 0.005 0.11 - 0.45 0.18 without unit of measurement Table 4 Sample Thickness Treatment Ice properties of the base metal + (mm) after rolling Pestigket test Notch test DWTT Stretch- tensile- 50% 85% strength- strength FATT SATT ity (kp / mm2) (kp / mm2) (° C) (° C) S 18.3 a 49.6 65.3 -110 -61 S 18.3 b 55.3 65.6 - 98 -53 S 18.3 c 57.0 73.8 - 95 -49 S 18.3 d 59.8 72.1 - 90 -46 S 18.3 e 63.6 73.3 - 84 -40 T 18, 3 a 72.4 81.9 - 93 -51 T 18.3 e 78.6 83.5 -119 -66 U 18.3 a 51.2 68.8 -103 -58 U 18.3 e 65.3 75.2 - 88 -43 *: direction of the test piece; crosswise direction table 4 (continuation) sample thickness treatment notch properties of severe cracking sensitivity (mm) step weld seam introduced heat 40 KJ / cm heat 100 KJ / cm I II VEo 50% - VEo 50% - (kg-m) FATT ( kg-m) FATT (i) (ii) (iii) (° C) (° C) S 18.3 a 15.5 -18 13.3 -5 0 50 # -15 S 18.3 b 14.2 -23 11.8 -7 0 50 # -15 S 18.3 c 16.7 -12 12.1 -4 1 50 # -15 S 18.3 d 12.1 -20 11.3 0 0 50 # - 15 S 18.3 e 13.9 -16 12.5 -9 2 50 0 T 18.3 a 11.8 -10 13.7 -1 5 75 0 T 18.3 e 12.2 - 8 11.4 -3 4 75 0 U 18.3 a 4.3 23 3.6 30 24 125 0 U 18.3 e 4.7 25 2.9 38 27 125 0 In the column "Sensitivity to welding fractures" in Table 4, the digits I or II represents the Battelle-type weld seam breakage test or the Y-slot weld seam breakage test, where (i) the weld seam breakage ratio (%) at the welding start temperature of oOC, (ii) the temperature preventing root breakage (9C), when a cellulose type electrode is used and (iii) a root breakage preventing temperature (OC) when a low hydrogen hydrogen electrode is used.

As can be seen from this Table 4, the properties are in the base metal and in the weld seam of the connection in the samples according to the invention S and T better than the comparative samples U. In particular, the advantages remain of the present invention in weld properties and weld fracture strength unchanged when the steel underwent various heat treatments after hot rolling has been subjected. It has thus been found that the steels according to the invention give excellent weldability.

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Claims (4)

Niobium-containing weldable structural steel with improved weldability Claims 1. Weldable structural steel containing niobium with improved weldability, consisting of 0.05 to 0.04% carbon, 0.01 to 0.50% silicon, 1.20 to 2.50 % Manganese, 0.01 to o, o7% niobium, o, oo5 to o, o30% titanium, o, oo5 to o, o6% aluminum, Remainder iron and unavoidable impurities, where Z C (%) + 10 N (%) 7 is not greater than 0.10% and Ti (%) / [C (%) + 10 N (%)] between o, o5 and o.6o is around the proportion of martensite islands in the welding heat-affected zone to be limited to no more than 15% of the area share. 2. Niobium-containing weldable structural steel with improved weldability, consisting of 0.05 to 0.04% carbon, 0.01 to 0.50% silicon, 1.20 to 2.50 % Manganese, o, o1 to o, o7% niobium, o, oo5 to o, o30% titanium, o, oo5 to 0.06% aluminum, at least one element in the prescribed quantity, selected from the group, those made of up to 0.50% Cu, up to 1.50% Ni, up to 0.50% Cr, up to 0.60% Mo, consists of up to 0.10 V, up to 0.03% B, up to 0.02% Ce and up to 0.03% Ca, Remainder iron and unavoidable impurities, where [C (%) + 10 N (% t7 not greater is as 0.10% and Ti (%) / [C (%) + 10 N (%)] is between 0.05 and 0.60 to the proportion on martensite islands in the welding heat-affected zone to no more than 15 % of the area share. 3. Niobium-containing weldable structural steel according to claim 1, characterized in that that this structural steel is used for line pipes. 4. Niobium-containing weldable structural steel according to claim 2, characterized in that that this structural steel is used for line pipes.