EP0181791A1 - Low-alloyed manganese steel for bicycle frame tubes, a frame tube produced and process for its manufacture - Google Patents

Abstract

1. Use, for the manufacture of cold-drawn weldless tubes for bicycle frames, of a low-manganese alloy steal consisting, by weight, of 0.16 to 0.20% of carbon, 1.20 to 1.50% of manganese, 0.15 to 0.50% of silicon, up to 0.035% of phosphorus, up to 0.035% of sulphur, up to 0.20% of nickel, up to 0.20% of chromium, up to 0.05% of molybdenum, 0.050 to 0.075% of vanadium, less than 0.10% of copper and than 0.010% of tin, optionally 0.007 to 0.030% of aluminium, the remainder consisting of iron and the usual impurities, whose proportion is kept at a very low level, the iron having, after a normalizing treatment, a fine structure containing dispersoids and a tensile strength higher than 700 N/mm**2 .

Description

The present invention relates to a low alloyed manganese steel for the manufacture of cycle frame tubes, a cycle tube made from this steel and the method of manufacturing this tube.

Known under the name AFNOR 20M5, a steel weakly alloyed with manganese, the composition of which is by weight: 0.16 to 0.22% of carbon, 1.10 to 1.40% of manganese. 0.15 to 0.35% of silicon, less than 0.035% of phosphorus, less than 0.035% of sulfur, the balance being formed by iron and the usual impurities.

Steels of the aforementioned type are used for tubes of cycle frames. These steels must be able to be brazed when the frames are manufactured. We seek to lighten the frames as much as possible and we need steels with maximum tensile strength.

Structural steels are also known which contain, in addition to manganese, a certain proportion of vanadium generally close to 0.10% or slightly higher. However, such structural steels have never been used for the manufacture of cycle tubes and their structures and properties have never been adapted for such use in the form of thin tubes which can be brazed for the manufacture of cycle frames.

The object of the invention is therefore to propose a steel slightly alloyed with manganese for cycle tubes which comprises, by weight, 0.16 to 0.20 Z of carbo ne, 1.20 to 1.50% manganese, 0.15 to 0.50% silicon, less than 0.035% phosphorus, less than 0.035% sulfur, up to 0.20% nickel, up to at 0.20 X chromium, up to 0.05% molybdenum, the balance being formed by iron and the usual impurities, the proportion of which is kept at a very low level, this steel having a high tensile strength, a very good ability to deform in hot and cold conditions, great resistance to fatigue and good brazing properties.

For this purpose, the steel according to the invention further comprises 0.05 to 0.075% vanadium and has, after normalization treatment, a structure dispersoids and a tensile strength su- ^p érieure _{700 N / mm} ² .

The object of the invention is also to propose, as a new product, a cycle tube manufactured with the steel according to the invention.

The object of the invention is also to propose a method of manufacturing such a cycle tube.

Other characteristics and advantages of the invention will emerge from the description which follows.

The steel according to the invention requires a particular production method which makes it possible to limit the content of impurities and residual elements to a minimum. In particular, phosphorus and sulfur are limited to very low values less than 0.035% and preferably less than 0.025%. For this, steel is produced by conversion to oxygen of a hematite cast iron, with stirring of the bath with an inert gas.

The steel obtained is then poured by the pro yielded from continuous rotary casting, in the form of round bars which thus have remarkable properties for the manufacture of seamless tubes.

The composition targeted for the steel is as follows: carbon: 0.18%, silicon: 0.35%, manganese: 1.30%, vanadium: 0.060%, phosphorus and sulfur: less than 0.025 Z.

Aluminum can be added in the form of wires, in an ingot mold, the targeted aluminum content being 0.020%. An aluminum content of between 0.007 and 0.030% will however be accepted. Aluminum stabilizes the fine-grained structure of steel, without having a deoxidizing role.

Preferably, with respect to the slightly wider intervals given, the steel according to the invention must contain: carbon: 0.16 to 0.19%. silicon: 0.33 to 0.40%, manganese: 1.28 to 1.35%. vanadium: 0.050 to 0.070%, aluminum: 0.010 to 0.030%, phosphorus and sulfur: less than 0.25%.

In addition, chromium should be limited to

0.20%, nickel 0.20% and molybdenum 0.050%.

The steel may also contain residual elements such as copper and tin in tolerated contents of less than 0.10% and 0.010% respectively.

To control the development and obtaining of the desired mechanical properties, samples are taken from certain portions of the casting bar, these samples then being forged and subjected to an analysis of their composition and also to a micrographic examination. Tensile strength is also measured on forged samples.

In the case of a steel according to the invention with 0.18% carbon, 0.35 / silicon, 1.3% manga- born, 0.060% vanadium and 0.020% aluminum, we observe on the forged samples, after normalization to 900 ° C and air cooling, a very fine structure with dispersoids which allows obtaining the desirable mechanical characteristics.

The tensile strength is close to ₈ 00 N / mm ² . This resistance compares very favorably to the tensile strength obtained with steels of the 20M5 type after quenching and tempering, this resistance being of the order of 620 N / mm ² .

The steels according to the invention which have contents of alloying elements included in the claimed ranges, after standardization treatment, have a tensile strength which is never less than 700-710 N / mm ² .

For the production of cycle frame tubes, the continuous casting bar is cut into sections with a minimum length of 2 m and then cooled.

The tube rings obtained can then be reheated and then transformed into seamless tubes by rolling. These tubes undergo a normalization treatment in the vicinity of 900 ^* which makes it possible to develop a fine structure with dispersoids and a tensile strength of the order of 80 ₀ N / mm ² .

These seamless tubes are then cold drawn to be transformed into thin cycle frame tubes. The considerable increase in the tensile strength of steel makes it possible to lighten the tubes, with equal resistance. This reduction is an appreciable advantage in the case of the manufacture of light weight cycles.

In addition to their very good tensile strength, the cycle tubes obtained have a very high resistance to fatigue and good brazing properties, due to the fact that they allow overheating of up to 1100 ° C. without altering their mechanical properties.

These properties make the steel according to the invention a high quality material for the construction of cycle frames.

Claims (6)

1.- Steel lightly alloyed with manganese for cycle tubes containing, by weight, 0.16 to 0.20% of carbon, 1.20 to 1.50% of manganese, 0.15 to 0.50 X of silicon, up to 0.035% phosphorus, up to 0.035% sulfur, up to 0.20% nickel, up to 0.20% chromium, up to 0.05% molybdenum, the balance being formed by iron and the usual impurities, the proportion of which is kept at a very low level, characterized in that it also comprises 0.050 to 0.075% of vanadium and that it exhibits, after normalization treatment, a fine structure with dispersoids and a tensile strength greater than 700 N / mm ² . 2.- low alloy steel according to claim 1, characterized in that it further comprises, 0.007% to 0.030% aluminum. 3.- low alloy steel according to claim 2, characterized in that it comprises 0.16 to 0.19% of carbon, 0.33 to 0.40% of silicon, 1.28 to 1.35% of manganese, 0.050 to 0.070% vanadium, 0.010 to 0.030% aluminum, less than 0.025% phosphorus, less than 0.025% sulfur, up to 0.20% chromium, up to 0.050% molybdenum, up to '' 0.20% nickel, less than 0.10% copper and less than 0.010% tin. 4.- Low alloy steel according to claim 3, characterized in that it contains substantially 0.18% carbon, 0.35% silicon, 1.30% manganese, 0.060% vanadium, 0.020% aluminum, less than 0.025% phosphorus and less than 0.025% sulfur. 5.- Tube for cycle frames with thin walls and good mechanical resistance, cold drawn, characterized by the fact that it is made of low alloy steel containing 0.16 to 0.20% of carbon, 1.20 to 1.50% of manganese, 0.15 to 0.50% of silicon, less than 0.035% of phosphorus and less than 0.035% of sulfur, as well as 0.050 to 0.075% of vanadium, with a fine structure with dispersoids having a tensile strength greater than _{700 N / mm} ² . 6. A method of manufacturing a cycle tube according to claim 5. characterized in that: - that a steel having the indicated composition is produced, from hematite cast iron, by conversion to oxygen with stirring with inert gas, - that this steel is poured in the form of round bars, by the rotary continuous casting process, with the addition of aluminum in the ingot mold, - that the bar obtained is cut in the form of tube rounds, - hot rolled tube rounds to produce seamless tubes, - that a standardization treatment is carried out on the seamless tubes, in the vicinity of 900 ° C., followed by air cooling, - cold stretch the seamless tubes to make thin cycle tubes.