EP0018425A1 - Construction steel with high fatigue resistance - Google Patents

Abstract

1. Constructional steel which has high fatigue strength, possesses good weldability up to a carbon content of 0.3% and is resistant to atmospheric corrosion, characterised in that in addition to iron it contains at most 1.6% (by weight) of C, 0.3 to 3.0% (by weight) of Mn and/or of Ni, at most 1.8% (by weight) of Si, 0.6 to 4.0% (by weight) of Cu, at most 3.0% (by weight) of Mo and/or Co, 0.02 to 0.4% (by weight) of Nb and/or of V, at most 0.006% (by weight) of B, at most 0.4% (by weight) of Zr and/or of Be, 0.02 to 0.2% (by weight) of Al, 0.005 to 0.2% (by weight) of N, at least 0.0001% (by weight) of Ca and at most 0.25% (by weight) of Ce and/or of Pb, and up to 0.1% of sulphur.

Description

The present invention relates to a structural steel having a high resistance to fatigue and, up to a well-defined carbon content, good weldability, and which is resistant to corrosion by air, this steel being in particular intended for the production of structures and frameworks, land or hydraulic works, vehicles, machines and machine components, infrastructure and superstructures for railways, etc., which are exposed to great cyclic stresses and bad weather .

The current economic situation, which makes it necessary in particular to achieve a general reduction in the consumption of energy and materials, places the whole industry, in particular in the fields of building, research and the production of hydrocarbons and transport, faced with technical and economic requirements which the properties of conventional steels can no longer meet, which, in a certain sense, slows down the development of these sectors.

Profitable development of construction and production methods and conventional technologies, as well as the application of new technical and technological solutions, and even also the exploitation of underground products which have not been developed until now for technical and economic reasons, are unthinkable if we do not have a new grade of steel with sufficient fatigue resistance and complex properties favorable for industrial processing, this grade of steel must be able be produced in large quantities and at a cost price low enough to be used very widely.

It therefore became essential to develop a new steel grade which, in accordance with these principles of saving energy and materials, could withstand the current stresses, the cross section of the construction, and therefore its own weight, being significantly lower, while offering greater security, and which is even capable of satisfying more demanding parameters, and of taking up the charges thus generated, the cost of industrial production and transformation of this steel should not more than exceed the specific costs incurred for the production of products manufactured with conventional steels.

Structural steels are already known which have good mechanical properties and good weldability when the conditions are appropriate.

In the field of weldable steels, the following steel grades can be listed, by way of example: T 1, RQC-100 A, HY and NAXTRA, from the United States, or HT, HW, KLN and RIVER-ACE, from Japan. The chemical composition of these steels is characterized by the following contents: 0.10 to 0.23% (by weight) of C, 0.50 to 1.50% (by weight) of Mn, 0.60 to 1.50 % (by weight) of Cr, and 1.0 to 9.5% (by weight) of Ni, and some shades additionally contain 0.50 to 1.00% (by weight) of Mo, 0.08 to 0 , 15% (by weight) of V, 0.003 to 0.04% (by weight) of B and 0.5 to 0.7% (by weight) of Cu.

It is characteristic of the mechanical properties of these steels that their apparent elastic limit - referred to an elongation of 0.2% - is between 500 and 700 N / mm ² , and that their plasticity lends itself to industrial transformation. Their fatigue resistance limit, in the case of a rupture occurring after 10 ⁵ stresses, is included, for a stress R = -1, between 200 and 400 N / mm ² and, for a stress R = 0, between 250 and 500 N / mm ² (on non-welded test pieces).

Some steel grades have some resistance to air corrosion.

The disadvantage of these steels is however that they can only be given their good resistance characteristics by a quenching and tempering treatment applied in special installations. Their mechanical properties are therefore the result of quenching and tempering, which limits the number of profiles that can be produced in this quality, also gives rise to great ins tability of these mechanical properties as a result of the lack of homogeneity of the quenching, and moreover results, due to the limited passage capacity of the installation, the.-complexity of the latter and the high costs which are incurred, by a manufacturing cost which reaches several times the cost price of normal steel production.

The state of the material which has undergone a quenching and tempering treatment constitutes an additional difficulty for industrial processing, in particular during hot cutting or cutting, making welded junctions and hot bending or bending.

The use of steels having undergone a quenching and tempering treatment is thus greatly limited, despite their favorable mechanical properties, and this as a result of the absence of the essential profiles, the lack of homogeneity of the mechanical properties, difficulties linked to their transformation and their high price. In the field of non-weldable materials, steels are also known which have excellent mechanical properties, such as, for example, the grades En and AISI-V, developed in the United States, or the grade GhNW, coming from from the Soviet Union, the shades Rex, Melt-A and HST, from Great Britain, or CSV4 and-MOG from the Federal Republic of Germany. Their chemical composition is characterized by the following contents: 0.2 to 0.6% (by weight) of C, 0.2 to 1.6% (by weight) of Si, 0.3 to 1.6% (in by weight) of Mn, 0.3 to 5.0% (by weight) of Mo and 0.1 to 1.0% (by weight) of V, but some shades also contain 1.5 to 3.0% (in by weight) of W and 0.1 to 0.3% (by weight) of Ti.

It is characteristic of the mechanical properties of these steels that their apparent elastic limit, for an elongation of 0.2%, is between 1300 and 1600 N / mm ² when they have undergone a quenching and tempering treatment, and their tensile strength is between 1700 and 2000 N / mm ² , which corresponds to an elongation of 7 to 10% and a resilience between 0.7 and 2 daJ / cm ² , on an Izod test piece not cut. For a stress R = 0, related to a number of cycles of ₁₀ ⁴ until rupture, their limit of resistance to fatigue is between 400 and 800 N / mm ² .

The disadvantage of these steels is that their properties, previously mentioned, only appear after a quenching and tempering treatment, which greatly limits their use due to processing difficulties (scale, scale, crosslinking or warping, degree machinability), and what also makes these steels quite fragile and sensitive to the notch effect, the cost of their development excluding more practically a large-scale industrial application, because of their high content of alloyed elements .

The currently known structural steels therefore have fairly good mechanical properties, both in the weldable field and in the non-weldable field, and this because of the alloy additions and heat treatments, that is to say say quenching followed by tempering. But this method of increasing the resistance limits the assortment of profiles which can thus be produced, the construction elements having undergone a quenching treatment can moreover hardly be machined using the usual machines, and finally, in the case of building elements which have undergone a transformation before quenching, the high quenching temperature causes decarburization, crosslinking or warping, and possibly cracking. The development of these steels requires special equipment, which further increases costs and does not allow a large-scale industrial application. The combination of these drawbacks leads to a substantial reduction in the useful value of these steels, despite their apparently favorable mechanical properties.

The object of the present invention is to develop structural steels resistant to wear and corrosion by air, and having good weldability up to certain carbon content limits (0.30%) steels of which fatigue strength limit and the apparent elastic limit are higher than those of conventional steels and which, thanks to their various reinforcement mechanisms and without quenching, serve as basic material for the realization of constructions and frameworks, earth or hydraulic works, vehicles, machines and machine components, which are exposed to heavy cyclic stresses and inclement weather.

The present invention achieves the objective set by the fact that the steel thus produced contains, in addition to iron, and the usual residual elements such as P, As, Se, etc., at most 1.6% (by weight) of C, 0.3 to 3.0% (by weight) of Mn and / or Ni, maximum 1.8% (by weight) of Si, 0.6 to 4.0% (in by weight) of Cu, at most 3.0% (by weight) of Mo and / or Co, 0.02 to 0.4% (by weight) 'of Nb and / or V, at most 0.006% (in by weight) of B, at most 0.4% (by weight) of Zr and / or Be, 0.02 to 0.2% (by weight) of Al, 0.005 to 0.2% (by weight) of N , at least 0.0001% (by weight) of Ca, and at most 0.25% (by weight) of Ce and / or Pb, sulfur can be present in certain cases up to 0.1%.

pour des aciers soudables.More particularly preferred compositions according to the invention comprise:

for weldable steels.

The preferred composition for non-weldable steels is as follows:

Some of the alloyed elements form, when they are in the report according to the present invention, complex metallic compounds which, in part, already produce, from the casting stage, active seeds of critical dimension, and which are also , in part, dissolved in the interstices thereby creating a prestress in the iron network and thus increasing the number of network faults. Other alloyed elements cause metallic precipitation with a high shear resistance, which increases and stabilizes at the same time, in a coherent way, the internal tension of the network of basic material.

The increase in the number of germs of critical dimension involves a strong increase in the aptitude for crystallization which the casting presents, a reduction in the time of solidification and the size of primary grain, an abrupt increase in the surfaces of the limits of the grains and a limitation of the possible formation of intermetallic enrichments.

The properties and the advantageous ratio of the components create, in the alloy system according to the present invention, thermodynamic, kinetic and germination conditions such, during dissolution, solidification, recrystallization and hot deformation, that the arrangement of the components for interstitial solution, the quantity of these components as well as the number and the degree of stress of the networks thus put under prestress are significantly increased.

Thanks to the increase in the number of networks presenting an interstitial prestress and their degree of constraint, the number of dislocations produced by metallurgical way and which favor and determine the formation, as well as the dispersion of metallic precipitations is greatly increased, which Significantly increases the efficiency of the anchoring or fixing function of precipitation during the dislocation front movement triggered by precipitation.

The components according to the present invention and their advantageous ratio thus automatically ensure the excellent metallurgical quality of the steel during its production and the positive effect of the various current reinforcement mechanisms, whose combined and cumulative action increases the useful mechanical resistance and the fatigue strength limit of steel.

The chemical composition of the steel according to the present invention also includes alloyed elements which do not dissolve in the iron and do not combine with it, but which enrich themselves on the surface of the steel. As a result, a dense protective layer which is difficult to dissolve and which protects the steel against the corrosive action of the environment and well defined fluids, eliminating the possibility of pitting corrosion and improving the color fastness of steel.

The steel according to the present invention has good weldability for a given carbon content and with an appropriate heat supply, and the properties of the heat affected zone are identical to those of the base material.

Since the production of the steel according to the present invention does not require a reducing atmosphere, it can take place using "conventional installations, and it is possible, by hot forming processes, to give the steel of any size and profile, by rolling or stamping, production can be done in large series without special installations.

The steel according to the present invention exhibits, without quenching, excellent mechanical properties, and at the same time allows the application of conventional transformation and assembly technologies.

In the field of non-weldable steel, the aptitude for transformation or machining, as well as the hardness after machining, can be adjusted by tempering by heat treatment at low temperature. The price of the steel according to the present invention is thus not burdened, as a basic material, by the cost of the complicated quenching and tempering treatment carried out in a special liquid, as well as by that of the installations necessary for this effect, and in addition the manufacturing costs of the products produced with the steel according to the present invention do not exceed the cost of conventional products.

This is why the benefit that can be obtained economically, from the technical advantages - offered by the steel according to the present invention (reduction of energy consumption and weight, etc.), thanks to the high limits of fatigue strength and elasticity, is practically unaffected due to the costs of developing and using the new base material.

The present invention will be better understood using the detailed description of several embodiments taken as non-limiting examples of the production of steel and its mechanical properties.

EXAMPLE 1

Three loads of steel according to the present invention are presented, by way of example, in the field of weldable steels. The charges were developed in 60 t arc furnaces and then refined in metallurgical equipment with pockets. The casting was carried out in a continuous casting installation with four dies having a profile of 240 x 240 mm, and was then produced by rolling, from the billets, and under normal conditions, steel rods of a diameter of 20 mm, which was then air-cooled on coolers.

The results of the load examinations according to the present invention appear below:

1.1. Chemical composition of the charges in percentages (weight)

Dans les exemples qui suivent les abréviations qui suivent ont les significations suivantes :

• Rp la limite élastique
• Rm la charge dë rupture
• A5 allongement
• Z striction
• KCU resilience

In the examples which follow the abbreviations which follow have the following meanings:

• Rp the elastic limit
• Rm the breaking load
• A5 elongation
• Z necking
• KCU resilience

1.2. Mechanical properties

1.3. Weldability

• Epaisseur de la plaque = V = 12 mm
• Type de soudure = contre-soudure (à un angle de 60°)
• Apport de chaleur = 3000 joule/cm
  
• Nombre de soudures = 3 + 1
• Atmosphère inerte = C0₂
• Fil à souder = le matériau lui-même, avec un diamètre de 1,6 mm

The test specimens, welded in an inert atmosphere, were examined for a 12 mm thick plate manufactured with load 2. The plate was not subjected to any heat treatment, either before or after welding.

• Plate thickness = V = 12 mm
• Type of weld = counter-weld (at an angle of 60 °)
• Heat input = 3000 joule / cm
  
• Number of welds = 3 + 1
• Inert atmosphere = C0 ₂
• Solder wire = the material itself, with a diameter of 1.6 mm

1.31. Tensile test

Rupture se produisant en dehors de la soudure

Failure occurring outside the weld

1.32. Plasticity of the thermally affected area

1.4. Resistance to air corrosion

(measured in the air volume of an industrial premises)

1.5. Fatigue resistance

The fatigue or endurance test was carried out on a fatigue test machine of the Schenk-Erlinger type, operating on the resonance principle. In this case, it is both the component of the static preload and the oscillating load (+ Fa) which are applied by springs resting on a common load head. The static load is established and regulated by a threaded axis and the oscillating spring is excited by an electric motor. The oscillation or vibration excited by the rotation of the eccentric mass activates the pulsator at the point of resonance, and said pulsator produces a static charge between 0 and 20 megabytes and a cyclic charge of + 10 mp.

A steel round, with a diameter of 20 mm, produced by rolling from load 2, was subjected to the fatigue test. The results of the control tensile test, carried out on a laminated test piece which has not undergone heat treatment, are shown in Table 5.

A titre de comparaison, l'essai a aussi été effectué sur la nuance d'acier 42CD4, selon la même méthode et sur une éprouvette similaire. La composition chimique de l'acier ayant servi de base de comparaison figure sur le tableau 6, tandis que ses propriétés mécaniques sont indiquées sur le tableau 5.

For comparison, the test was also carried out on steel grade 42CD4, according to the same method and on a similar test piece. The chemical composition of the steel used as a basis for comparison is shown in Table 6, while its mechanical properties are shown in Table 5.

1.51. Load levels of the fatigue test

1.52. Stress corresponding to the degrees or steps of load and to which the test pieces have been subjected

1.53. Result of the fatigue test

1.54. Interpretation of the results of the fatigue test

By comparing the test results obtained with an identical stress of the steel 42CD4 and the steel of the load 2 produced according to the present invention, it can be seen, for the probability of failure of 50%, that this value corresponds 60,000 stresses in the case of steel used as a basis for comparison, against 700,000 stresses in the case of steel according to the present invention. The comparison of the results obtained by identical test methods shows that with identical load the lifetime of the steel according to the present invention is approximately equal to ten times that of conventional steel serving as a basis for comparison.

By comparing the values of resistance or the loads of the duration correspond to the probability of rupture of 50%, that is to say the lines which represent, in the same figure, the resistance to fatigue of the two materials, one notes that the steel according to the present invention supports loads which are almost double that of the steel 42CD4.

EXAMPLE 2

Two charges constituted by steel according to the present invention are presented, by way of examples, in the field of non-weldable steels. The charges were produced in an arc furnace of 65. t, then refined in metallurgical equipment comprising pockets, and poured into a continuous casting installation having a profile of 240 x 240 mm. Steel rounds were then produced, by rolling, under normal conditions, from the rods, and cooled on coolers. The diameter of these steel rounds was 20 mm. The results of the tests are shown below.

2.1. Chemical composition of the charges

2.2. Mechanical properties

2.3. Fatigue resistance

The purpose of the fatigue test was to examine the properties of the steel according to the present invention when it is subjected to an oscillation or vibration force varying with time. The test method used was, in addition to the fatigue tests by cyclic torsional forces with the usual torsional fatigue test specimens, the locati method, intended to determine the resistance to combined bending and torsional forces, and the we finally calculated the resistance to oscillations or vibrations by processing the results on computer tor. For the load fatigue test 4, test pieces made from rolled steel rods and subjected to a stress relieving treatment, with a diameter of 40 mm, were used. The results of the mechanical.static test of the steel rods produced by rolling from the load 4 are shown in Table 13.

2.31. Fatigue test by cyclic torsional forces

The purpose of this test was to determine the Wöhler diagram for the combined effort of bending and symmetrical oscillation.

2.32. Degrees or steps of load of the fatigue test by cyclic torsional forces

2.33. Parameters of the fatigue test by cyclic torsional forces

2.34. Test result

2.35. Data relating to the distribution of the results of the fatigue test by cyclic torsional forces, after processing of these results by the computer.

2.36. Torsional stress test

The purpose of this test was to determine the Wöhler diagram for the combined force of torsion and symmetrical oscillation.

2.37. Degrees or steps of load of the torsional stress test

2.38. Parameters of the torsional stress test

2.39. Result of the torsional stress test

2.4. Dynamic or oscillatory or vibration resistance values determined on the basis of the fatigue test by cyclic torsional forces

2.5. Resistance to oscillations or vibrations determined on the basis of the torsional stress test

2.6. Results interpretation

The resistance values to oscillations or vibrations which have been obtained, ie Rvh = 373 to 441 N / mm ² and Tv = 254 to 255 N / mm2, with a steel ring produced from the steel according to the present invention , not having undergone a quenching treatment followed by tempering and with a diameter of 40 mm, are consistent with the values of resistance to oscillations or vibrations of known spring steels having undergone quenching or a quenching treatment followed by returned. It should be noted that during the fatigue tests by cyclic torsional forces, it was possible to obtain a significant improvement in the resistance values to oscillations or vibrations by an appropriate preliminary charge, of the order of several millions, which was produced by a stress of around 440 N / mm ² . The resistance to oscillation or vibration values of the steel according to the present invention therefore improve appreciably when it has been put in place in a construction, as a result of the work of the frameworks, which constitutes a very useful property of the steel according to the present invention.

Claims (4)

1 - Structural steel having a high resistance to fatigue and, up to a well-defined carbon content, good weldability, and which is resistant to corrosion by air, characterized in that it contains, in addition iron, maximum 1.6% (by weight) of C, 0.3 to 3.0% (by weight) of Mn and / or Ni, maximum, 1.8% (by weight) of Si, 0.6 to 4.0% (by weight) of Cu, maximum 3.0% (by weight) of Mo and / or Co, 0.02 to 0.4% (by weight) of Nb and / or of V, at most 0.006% (by weight) of B, at most 0.4% (by weight) of Zr and / or Be, 0.02 to 0.2% (by weight) of Al, 0.005 to 0 , 2% (by weight) of N, at least 0.0001% (by weight) of Ca, and at most 0.25% (by weight) of Ce and / or Pb, and up to 0.1% sulfur. 2 - Structural steel according to claim 1, characterized in that it has the following composition in addition to iron and certain residual elements

3 - Structural steel according to claim 1, characterized in that it has the following composition of iron and certain residual elements:

4 - Process for manufacturing a structural steel having a high resistance to fatigue and up to a well-defined carbon content, good weldability and which is resistant to corrosion, characterized in that it is produced in a furnace a charge comprising, in addition to iron, at most 1.6% (by weight) of C, 0.3 to 3.0% (by weight) of Mn and / or Ni, at most 1.8% (in weight) of Si, 0.6 to 4.0% (by weight) of Cu, maximum 3.0% (by weight) of Mon and / or Co, 0.2 to 0.4% (by weight) Nb and / or V, maximum 0.006% (by weight) of B, maximum 0.4% (by weight) of Zr and / or Be, 0.02 to 0.2% (by weight) of Al, 0.005 to 0.2% (by weight) of NO at least 0.0001% (by weight) of Ca, and at most 0.25% (by weight) of Ce and / or Pb, the sulfur may be present in some cases, up to 0.1% which is then refined in metallurgical equipment equipped with pockets, then casting, rolling and cooling.