CZ278435B6 - Process of heat treatment of cold rolled plain carbon and micro-alloyed low-carbon steels - Google Patents

Abstract

The procedure described in this patent is related to the thermal processing of cool rolled band steel used in the production of the bond tape, while the above mentioned steel contains 0,35 weight percent of carbon, 1,3 weight percent of manganese, or titanium, aluminium, niobium, less than 0,5 weight percent and these elements represent micro alloy compound, iron and accompanying elements together with impurities represent the residual compounds. The basic principles of this procedure is related to that, the steel is being undertaken to the thermal processing, at the temperature of 300 up to 470 Celsius degrees, for 10 seconds up to 12 hours. The heating time for the appropriate heating temperature is given by the interval, the upper border of which is determined by the elongation growth and the upper one is related to the tenacity decrease under the reinforcement limit, created based on the cool shaping, and after that the material is being cooled quickly, it means with the velocity of 1 up to 900 Celsius degrees per minute.

Description

Method of heat treatment of cold formed unalloyed and microalloyed low carbon steels

Technical field _

The invention relates to a method of heat treatment of cold formed unalloyed and microalloyed low carbon steels, in particular cold rolled strip steels for the production of a binding tape.

BACKGROUND OF THE INVENTION

For binding coils, crates, lump materials and the like, a steel binding tape made from cold rolling of non-alloy low carbon steel, containing by weight not more than 0,35% of carbon, * 1,3% of manganese or 0,5% of titanium, is used , 0.5% aluminum, 0.5% niobium, the rest iron and other accompanying elements and impurities. Cold rolling leads to material hardening caused by increased dislocation density and slippage blockage. Outwardly, this strengthening results in an increase in strength, hardness, yield strength and reduced ductility and notch toughness. Thus, in the manufacture of the steel strap, its required strength of about 700 to 800 MPa is achieved, but its disadvantage is the low ductility of about 4, which causes cracking in use and limits its applicability. The ductility can be increased by recrystallization annealing at temperatures of 550 to 700 ° C. The disadvantage of this annealing is the fact that as the ductility is restored, the desired hardening is also removed.

SUMMARY OF THE INVENTION

This disadvantage is eliminated by the heat treatment method of cold-formed non-alloyed and low-carbon steels according to the invention, which consists in subjecting the steel to a temperature of 300 to 470 ° C for 10 seconds to 12 hours, each for a single heating temperature. the heating time is given by a time interval whose lower limit is determined by an increase in ductility and the upper limit by a decrease in strength below the limit of the hardening then obtained by cold forming is cooled at a rate of 1 to 900 ° C / min.

An advantage of the heat treatment process according to the invention is that both the ductility of the steel is increased and the strength obtained by the previous cold forming is maintained, or the strength is increased. For example, the ductility is increased from 3% to 8%, which is sufficient for use with a steel strap, and with an optimal combination of the heating temperature and the time the steel is exposed to it, the steel strength is also increased from about 800 The process of heat treatment according to the invention can be carried out both in stationary furnaces and in continuous furnaces, the heat treatment of the expanded steel strips in the continuous furnaces is carried out for a period of time from the lower part of the time interval and the processing of steel strips in coils is done for a period of time from the upper part of the time interval. In the case of heat treatment in air, the accompanying phenomenon is the formation of oxides, which increase the corrosion resistance of the steel, while the surface of the tape creates a lead-in paint, which is

-1GB 278435 B6 indicates the optimum relationship between temperature and heat treatment time.

DETAILED DESCRIPTION OF THE INVENTION

In particular cases, the optimum combination of temperature and annealing time depends on the type of material, the continuous annealing of the expanded strips on the strip thickness, the annealing in coils to the charge size and the type of furnace, atmosphere used and the like. It is therefore not possible to automatically apply the annealing parameters at which one of the furnaces has achieved successful results for another furnace. The entire heat treatment process takes place at a certain temperature and time interval, which is the narrower the higher the temperature is used. Therefore, it is advantageous to carry out annealing in a continuous manner on a developed web at temperatures above 400 ° C.

To find the optimum parameters, the following procedure must be followed. Test strips are made from a strip prepared for heat treatment, where the tensile strength and ductility are determined by tensile testing. This is repeated after the heat treatment has been carried out. If the breaking strength does not change or changes only slightly and the ductility stays at the same level or even decreases, a low temperature or a short annealing time has been used and the temperature or dwell time or both need to be increased. If the ductility of the material increases, but its strength decreases more than desired, the temperature or the annealing time were too high and either the temperature or the annealing time or both need to be reduced. If the ductility increases above the desired value but the strength is maintained at its original level or only increases or decreases to a value that is even higher than the required minimum for a given material, the correct annealing parameters are achieved. Minor corrections of the annealing time or temperature can only be made to emphasize the ductility or strength of the annealed material.

Example 1

According to a first embodiment of the heat treatment process according to the invention, the steel containing 0.24% by weight of carbon, 0.24% by weight of manganese, 0.11% by weight of phosphorus and sulfur, the remainder of the iron was exposed to 300 ° C in air in a stationary bell furnace. for 8 hours. It was then cooled at a rate of 100 ° C / h. In the initial state after cold rolling, the steel strip showed a strength R _m equal to 815 MPa and an elongation A _{5 of} 3.5%. After the heat treatment according to the invention, the ductility A ₅ increased to 7% and at the same time the strength increased by 20 MPa to 835 MPa.

Example 2

A similar type of steel, containing by weight 0.19% carbon, 0.03% phosphorus, 0.04% sulfur, the remainder iron, manganese and silicon was exposed in a continuous heating furnace at 400 ° C for 4 minutes under a protective atmosphere % by volume of 5% hydrogen and 95% nitrogen. It then cooled freely at 400 ´C per minute. Steel strip which had before this heat treatment in

Also, after cold-rolling, the strength R _m of 815 MPa and the elongation A ₅ equal to 4%, after heat treatment under these conditions, exhibited a strength R nj of 855 MPa and an elongation of Ag of 8%.

Example 3 '

A similar type of steel, containing by weight 0.19% carbon, 0.03% phosphorus, 0.04% sulfur, the remainder iron, manganese and silicon, was exposed in a continuous heating furnace at 450 C for 20 seconds without the use of protective atmosphere. It was then cooled in air to ambient temperature over 1 minute. A steel strip which, before this heat treatment, had in the initial state after cold rolling R _m equal to 815 MPa and an elongation A ₅ equal to

After heat treatment under these conditions showed a strength of 820 MPa and ductility Ας 9%.

Example 4

Against Example 1, where the charge was 250 kg, at a charge of 5 t, the steel contained 0.21% carbon, 0.20% manganese, by weight. 0.10% of phosphorus and sulfur, the remainder iron, is annealed in coils in a stationary hood furnace under a nitrogen atmosphere at 380 ° C for 10 hours. Then the coils were cooled at an average rate of 50 ° C per hour. In the initial state after cold rolling, the steel strip showed a strength R _m equal to 875 MPa and an elongation A ₅ equal to 3%. After heat treatment, the ductility A ₅ increased to 7 to 11% while maintaining a strength in the range of 870 to 895 MPa.

Industrial applicability

The heat treatment process according to the invention can also be applied to other products of unalloyed and microalloyed low carbon steels, which require increasing their useful properties by improving the mechanical values of the material in terms of both high ductility and high strength obtained by prior cold forming.

Claims (1)

Method for heat treatment of cold-formed unalloyed and microalloyed low-carbon steels, in particular cold-rolled strip steels for the production of binding bands, containing by weight not more than 0.35% carbon, 1.3% manganese or titanium, aluminum and niobium up to 0, 5% as microlegur, the remainder iron and the accompanying elements and impurities, characterized in that the steel is subjected to a temperature of 300 to 470 ° C for 10 seconds to 12 hours, the time of heating being determined for each individual heating temperature. the lower limit of which is determined by the increase in ductility and the upper limit by the decrease in strength below the limit of the cold-hardening, then cooled at a rate of 1 to 900 ° C / minute.