High Tensile Steel for Tires

A full set of tyres is a major constituent of any car or truck because it has the exclusivity of contact with the road. Progress in automotive technology is impossible without synchronous improvement in tyre developments.

Types of Tyres

Although the tyre appears to be a simple thing, it is, in fact, a complex composite with as demanding a combination of requirements as any part of the vehicle structure. Since the introduction of the first steel-reinforced tyres, a continuing series of innovations has enabled cold drawn 0.8% C steel cord to remain the most economical and efficient way to manufacture high performance tyres in mass production. Consequently the market share for textile and glass fibres has been reduced or reserved for specific niches.

Factors Leading to Increased Life of Tires

The aims have been to reduce the unsprung mass of the vehicle, with consequential improvements in fuel economy and reduced emissions, and to increase the lifetime of tyres. These have been achieved by increasing the strength of steel cord filaments by 15%, with a corresponding increase in fatigue performance as well as maintaining the level of ductility of individual filaments and optimising manufacturing methods to control costs. A change in the design of the cord assembly was made in order to improve the reinforcement functionalism of the tyre.

Why use Steel?

Steel is the only material available to reinforce tyres which has a stabilised endurance limit, i.e. below a characteristic stress level no fatigue crack propagation will occur with infinite fatigue life. Above this threshold stress level, the higher strength steel cord gives between 10 and 30% longer life and enables truck tyres to be safely retreaded twice, giving lives of 500,000 km.

Steel Developments

A new high strength steel has been developed through the adoption of continuously casting with low segregation levels and high surface quality. Modifications and improvements to the drawing and heat treatment equipment have enabled steels with higher carbon contents to be drawn. The cold work deformation ψ increased from 3.2 to 3.5. The availability of the higher strength steel has facilitated the use of more productive stranding equipment. New strand and cable assemblies have been developed to make it possible to transmit an increased shear stress from the reduced cord surface to the surrounding rubber.

The strength of steel cord filaments is related to the logarithmic value of the filament size, table 1.

Table 1. Strength of steel cord filaments in relation to filament size.

The amount of cold work is limited to levels that assure an acceptable ductility, and this is size dependent (figure 1).

Substitution of a high tensile steel cords for the normal cords has resulted in a projected weight reduction of up to 16% for both truck and passenger tyre varieties (figure 2). This weight reduction has come with no loss of ply strength.

Load Transfer

As far as tyre design is concerned, the increase in the ultimate strength of the filaments must be effectively transferred to the functional surroundings of the reinforcement. To increase its efficiency the coated cord ply must have a lower weight for the same strength or a higher strength for the same weight. Higher strength cord also requires less special rubber providing adherence to the brass-coated cord.

Implications of High Tensile Cords

Because the wall thickness of the tyre body is decreased with the higher strength steel cord the hysteresis loss of deformation energy is also reduced, leading to lower fuel consumption and lower running temperature. The overall benefits include the following:

•         Reduced weight tyres

•         Reduced unsprung mass

•         Longer life tyres

•         Reduced fuel consumption

•         Reduced vehicle emissions

•         Reduced running costs

•         Lower consumption of natural resources.

Primary author: Josef Lombaerts

Source: Materials World, Vol. 5 no. 6 pp. 331-32 June 1997.