Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60

Definitions

• the invention relates to a use of a self-tempering, low-carbon steel for springs, especially leafsprings for vehicles, the steel having a low carbon content and an addition of boron for improved full hardening and for making the steel self-tempering.
• helical springs are most commonly made of round material and the resilient properties of the steel from which they are made are achieved above all by cold working such as drawing of the material.
• springs having a larger cross-sectional area such as leafsprings of various designs.
• leafsprings Apart from many other advantages it is a characteristic of the leafsprings that their stability in other directions of motion than the one in which the spring is intended to act is high.
• the moment of forces acting on leafsprings and being greatest at their centre, that is where one provides the largest cross-sectional area can be taken up in different ways.
• the spring leaf can be made thicker at the centre. Such springs are often referred to as parabolic leafsprings and have recently been gaining importance. Parabolic leafsprings require relatively little space but on the other hand their production is costly.
• the most common type of leafsprings is the so-called laminated leafsprings and above all the type produced from flat bars, which are cut to different lengths and stacked on top of one another. The top leaf is longest and has fastening eyes at the ends.
• the length of the leaf is then reduced towards the centre, and the leaf package is held together on the one hand by means of a pin passing through the centre and on the other hand by a number of yokes placed about the package between the centre and the fastening eyes.
• These springs are cheapest from the point of view of production. Owing to the mutual friction between the leaves the spring action is also subject to hysteresis, which causes the motion of the spring to be attenuated.
• a good product for the production of springs has to satisfy stringent requirements of repeatability in order to avoid a lack of symmetry in the spring action. Apart from small dimensional variation also the variation in the characteristics of the steel should be small.
• the most important characteristics of steel used for springs are a high elastic limit and a high fatigue limit.
• the energy storing capacity of the spring material is proportional to the square of the elastic limit (R E 2 ).
• elastic limit is meant the maximum specific loading to which the material can be exposed for its return to the initial position without deformation of the material. With many types of steel the values of the elastic limit and the yield stress are almost identical.
• the fatigue limit follows in principle the elastic limit but, in addition, depends above all on the structural homogeneity and the surface finish, which benefits from a smooth surface free of decarbonisation layers and defects such as slag inclusions etc.
• a full hardening steel is indicated, i.e. a steel with a composition so adjusted to the thickness of the material that quenching results in the production of martensite also at the centre of the section.
• the material is tempered at temperatures generally in the region of 400-500°C in order to achieve a certain degree of toughness.
• such tempering causes the elastic limit to drop.
• SS 2090 contains 0.52-0.60% C and 1.5-2.0% Si.
• Si used by way of alloying constituent is relatively expensive.
• SS 2230 contains 0.48-0.55% C, slightly more silicium as well as 0.70-1.00% Mn, 0.90-1.20% Cr and 0.10-0.20% V. This steel is used for relatively large springs. The increased contents of silicium, manganese and chromium contribute above all to the hardenability whereas vanadium is added for grain refinement.
• the product of the titanium content and of the nitrogen content is 0,00125, which can be figured out of this steel.
• the steels mentioned are produced in an electric arc furnace, treated with vacuum and by a stream of argon and use to have low contents of phosphorus. This method of steel production also is known for minimising oxygen contents. However the use of this steel. for the production of hot rolled springs, especially leafsprings, is not known.
• the invention provides the use of a self-tempering, low-carbon steel for springs, especially leafsprings, the steel having been produced by processing a steel melt in an oxygen converter with a degree of blowout and a lance guiding system such that the nitrogen content at the point of tapping the steel melt into the ladle does not exceed 20 ppm, the melt at the point of tapping having the following composition:
• silicium-calcium containing not less than 20 per cent by weight of metallic calcium Prior to the vacuum treatment silicium-calcium containing not less than 20 per cent by weight of metallic calcium is blown into the melt.
• the basic material for the production of springs according to the invention is an unalloyed carbon steel having the following composition:
• the C content is kept lower than in a conventional spring steel due to the addition of boron.
• This of course makes the steel cheaper, and at the same time it becomes less liable to surface decarbonisation which would cause the fatigue limit to be reduced.
• this low-carbon steel which contains boron can be easily machined by means of cutting and shearing tools even in the hot rolled and non-annealed condition, whereas a conventional spring steel must be annealed owing to the high carbon content or machined in the hot state. With soft annealing there is also an increased risk of surface decarbonisation.
• boron causes the steel to be self-tempering, i.e. a toughness suitable for its use in springs is achieved directly after hardening.
• a toughness suitable for its use in springs is achieved directly after hardening.
• the toughness value (Charpy W) is measured at room temperature it varies between 25 and 25 J.
• tempering at max. 300° and preferably max. 230°C may be effected in order to increase the impact strength or homogenise the material characteristics.
• the vacuum process enables on the one hand a low oxygen content by degassing and slag separation while at the same time making possible to apply such control methods that the absorption of nitrogen from the air is reduced to a minimum.
• the steel is further deoxidised in the usual manner by adding aluminium so that uncombined oxygen in the melt does not exceed 40 ppm and preferably 15-20 ppm.
• the oxygen content is limited on the one hand because uncombined oxygen has a detrimental effect on the quality of the steel, and on the other hand because the affinity of titanium to oxygen is higher than its affinity to the residual nitrogen, which it is intended to bind.
• titanium is added in order to render such nitrogen as occurs in the melt as harmless as possible by causing it to combine so as to give rise to titanium nitride.
• the addition of titanium to an extent of 0.015-0.050% by weight has proved suitable, the most advantageous range being 0.020-0.045.
• a disadvantage consists in the fact that titanium nitrids formed with this process constitute inclusions which may effect the characteristics of the steel, inter alia as regards cuttability and fatigue strength.
• the nitrogen content of the steel should be as low as possible.
• the steel is cast in a continuous casting plant using the method of protected casting, i.e. with the aid of ceramic protective pipes to surround the stream or subject to the provision of another protective means about the casting streams, where the steel passes through air. In this way direct contact between steel and air is avoided, which may cause nitrogen and oxygen to be absorbed. If necessary use is made of a protective atmosphere.
• Reduction rolling, from the cast melt to the finally rolled product, is also of great importance especially as regards the fatigue limit of a spring steel.
• the area must be reduced by at least 22:1 in order to obtain a well processed steel.
• the slag residues still left over in the steel may after rolling be deformed to such a shape as to exert a negative effect on the fatigue and strength characteristics.
• the melt may be pretreated with metallic calcium by blowing powder into the melt, especially in the form of so-called silicium-calcium containing at least 20% by weight of metallic calcium. This is best done prior to the vacuum treatment. This results on the one hand in the amount of sulphide slags in the steel being reduced to a minimum and on the other hand in the remaining sulphide slags being modified after rolling to a less dangerous, round shape.
• the disadvantage consists in the fact that this process may lead to an increased content of nitrogen.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Articles (AREA)
• Medicines Containing Material From Animals Or Micro-Organisms (AREA)
• Heat Treatment Of Steel (AREA)
• Springs (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (2)

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Family Applications (1)

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• 1982
  • 1982-09-06 SE SE8205037A patent/SE8205037L/sv not_active Application Discontinuation
• 1983
  • 1983-09-05 AT AT83902908T patent/ATE32528T1/de not_active IP Right Cessation
  • 1983-09-05 DE DE8383902908T patent/DE3375699D1/de not_active Expired
  • 1983-09-05 EP EP83902908A patent/EP0139649B1/en not_active Expired
  • 1983-09-05 WO PCT/SE1983/000310 patent/WO1984000981A1/en not_active Application Discontinuation
• 1984
  • 1984-10-05 FI FI843928A patent/FI73467C/sv not_active IP Right Cessation

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