EP0018425B1 - 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, can 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 loads thus generated, the cost of industrial production and transformation of this steel not having to be moreover not 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, HYet 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 be given their good resistance characteristics only 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 instability of these mechanical properties due to the lack of uniformity of quenching. , and moreover, due to the limited passage capacity of the installation, the complexity thereof, and the high costs which are incurred, by a manufacturing cost which reaches several times the cost price of normal steel making.

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 grades 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 1,300 and 1,600 N / mm ² when they have undergone a treatment of quenching and tempering, and that their tensile strength is between 1,700 and 2,000 N / mm ² , which corresponds to an elongation of 7 to 10% and a resilience of between 0.7 and 2 daJ / cm ² , on a non-notched Izod test tube. 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 due to the alloy additions and heat treatments, that is to 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 production of these steels requires special equipment, which further increases costs and does not allow 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.

Also known from patent SU-A-570657 GOLOVIN is a steel containing carbon, manganese, silicon, copper, nickel, vanadium, molybdenum, aluminum, calcium, nitrogen and possibly cerium. , niobium, boron.

The proportions of berylium, zirconium, lead and cobalt are different from the proportions used in accordance with the invention.

Furthermore, the products according to this document have relatively low elastic limits compared to the steels according to the invention.

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 the limit of resistance to fatigue and the apparent limit of elasticity are higher than those of conventional steels and which can, thanks to their various reinforcement mechanisms and without quenching, serve as basic material for the realization of constructions and frameworks, d '' land or hydraulic works, of vehicles, machines and machine parts, which are exposed to great cyclic stresses and bad 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% (by weight ) of B, at most 0.4% (by weight) of Zr and / or Be, 0.02 to 0.2% (by weight) of AI, 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 the 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.

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 any dimensions and profiles, by rolling or stamping, production can be done in large series without special facilities.

The process in accordance with the invention consists in developing in a furnace a charge comprising, in addition to iron, the elements defined above, then in refining in metallurgical equipment provided with pockets, then in casting, or rolling and on cooling.

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 that of the installations necessary for this. Indeed, 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 in energy consumption and weight, etc.), thanks to at 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 ont les significations suivantes :

• Rp la limite élastique
• Rm la charge de rupture
• A5 allongement
• Z striction
• KCU résilience.

In the examples which follow, the abbreviations 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 = 3 000 joule/cm Vmm
• Nombre de soudures = 3 + 1
• Atmosphère inerte = CO₂
• Fil à souder= le matériau lui-même, avec un diamètre de 1,6 mm

The test specimens, welded in an inert atmosphere, were examined on 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 = 3,000 joule / cm Vmm
• Number of welds = 3 + 1
• Inert atmosphere = CO ₂
• Solder wire = the material itself, with a diameter of 1.6 mm

1.3.1. Tensile test

• R ^0.002 = 784.7 N / mm2
• R _m = 902.6 N / mm ²
• A5 = 16%
• Z = 52%

Failure occurring outside the weld

1.3.2. Plasticity of the thermally affected area

1.4. Resistance to air corrosion

Schenk-Erlinger, fonctionnant d'après le principe de résonance. Dans ce cas, ce sont à la fois la composante de la précontrainte statique et la charge oscillante (± Fa) qui sont appliquées par des ressorts s'appuyant sur une tête de charge commune. La charge statique est établie et réglée par un axe fileté et le ressort oscillant est excité par un moteur électrique. L'oscillation ou vibration excitée par la rotation de la masse excentrique actionne le pulsateur au point de résonance, et ledit pulsateur produit une charge statique comprise entre 0 et 20 mégapond et une charge cylique de ± 10 mp.(measured in the air volume of an industrial premises)

Schenk-Erlinger, operating on the principle of resonance. 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 resonance point, and said pulsator produces a static charge between 0 and 20 megapond and a cylindrical 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.

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.

1.5.1. Load levels of the fatigue test

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

1.5.3. Result of the fatigue test

1.5.4. 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 loads in the case of steel used as a basis for comparison, against 700,000 loads in the case of steel according to the present invention. The comparison of the results obtained by identical test methods shows that, with an identical load, the service life of the steel according to the present invention is approximately equal to ten times that of conventional steel used as a basis for comparison.

By comparing the resistance values or the loads of duration corresponding 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, it is found 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 a 65 t arc furnace, then refined in metallurgical equipment with pockets, and poured into a continuous casting installation with 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 the resistance to oscillations or vibrations was finally calculated by processing the results on a computer. 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 static mechanical test of the steel rods produced by rolling from the load 4 are shown in Table 13.

2.3.1. 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.

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

2.3.3. Parameters of the fatigue test by cyclic torsional forces

2.3.4. Test result

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

2.3.6. Torsional stress test

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

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

2.3.8. Parameters 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 (See table 22 p. 12)

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 / mm ² , 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, agree with the resistance to oscillation or vibration values of known spring steels having undergone quenching or a quenching treatment followed by tempering. 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. 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 weig ht) 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.

2. 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 containing iron and certain residual elements it has the following composition :

3. 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 containing iron and certain residual elements it has the following composition :

4. Process for the manufacture of a 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 a charge which in addition to iron comprises at most 1.6 % (by weight) of C, 0.3 to 3.0 % (by weight) of Mn and/or 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, 0.2 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 0.000.1 % (by weight) of Ca and at most 0.25 % (by weight) of Ce and/or of Pb, with sulphur being present in certain cases in an amount of up to 0.1 %, is processed in a furnace, that it is thereafter refined in metallurgical equipment provided with Ladles and that it is thereafter cast, rolled and cooled.