DE2021245C3 - Continuous heat treatment process on rod-shaped structural steels - Google Patents

Description

The invention relates to a continuous Heat treatment process on rod-shaped structural steels to improve the mechanical material properties, such as tensile strength, using rapid heating with subsequent quenching.

The inductive method is particularly suitable for rapid heating. The basics for this belong to the state of the art and are described in connection with application examples in "Basics of inductive heating" (Das Industrieblatt, Stuttgart, April 1960, pp. 204/10). The best known area of application is surface induction hardening. In this process, high-carbon (hardenable or heat-treatable) steel grades are obtained briefly heated in the bowl and then deterred As continued from the reference is evident, but it is also possible by means of induction to achieve as uniform a heating of the workpieces as possible (forge heating). Hereinafter Known methods are recognized that use rapid heating by means of induction.

From the German patent 8 79 111 it is known to electroinductively temper a steel suitably hardened from the rolling heat.

ίο According to the German patent specification 9 25 175 - to achieve steel workpieces with zones of different hardness - in the compensation of the Quenching process interrupted in the meantime in order to allow the quenched outer zone to be started from the inside.

Another quenching and tempering treatment in which a steel wire is fully austenitized when heated, Quenched and then tempered in a special way is described in the Austrian patent specification 1 73 474 described.

All of these applications relate to high-carbon, non-weldable steel grades, in particular on quenched and tempered steels.

In the heat treatment of reinforcing steel, a method with the preamble of claim 1 specified characteristics became known in the following context:

According to Evangulova, E. P, Golovin, G. F. (»Surface hardening of concrete steel«, pocket book for employees of the All Union Research Institute for High Fre-

jo quenztechnik, "Maschinenbau", Moscow-Leningrad, 1965, part 6, pp. 92 to 100), a reinforcing bar in the rolled state with a diameter of 32 mm (0.26% carbon; 035% manganese) in the shell is set at 970 to 980 ⁰ C quickly warmed up, then

v, quenched and then re-annealed at a temperature of 500 to 530.degree. C. Measurements showed that the core was heated to a temperature of 600 to 650.degree. Tests in which the back annealing was dispensed with showed that with satisfactory strength and Yield limit values, the elongation values not the norm were enough.

Finally, reference should be made to the German Auslegeschrift 12 46 002, according to which a in the rolled state Present reinforcing steel from an inductive annealing treatment Treatment of the near-surface cross-sectional ^{parts can be subjected to a temperature between 600 and 1050 0} C with subsequent quenching and subsequent cold deformation by twisting, stretching or the like.

w In the case of the two last-mentioned prior publications: the principle of rapid heating with subsequent quenching is an excerpt from the mandatory process steps. The subject of the application is subject to the following considerations

Y, based on: It is known that the temperature and time are decisive for the result of a heat treatment. At temperatures below Ai, the crystals initially recover - in this case there are still no changes in the structure

To recognize Mi - or to a new formation of the crystals (Recrystallization) via nucleation and nucleation growth. This will make the solidifications so far desired, dismantled, the technological properties adapted to the purpose or for the

hi further processing required malleable state restored. During the heat treatment, the tensile strength and the yield strength decrease, the yield strength preferably falling more while the

Elongation increases In addition to crystal recovery and recrystallization, the possible formation of Precipitations for changing the technological properties are taken into account.

The opposite change in tensile strength and yield point, on the one hand, and elongation at break, on the other, is also evident in the tempering of hardened steels with a higher carbon content. The aim of tempering is a high yield point and high tensile strength, ie a high yield point ratio, with good toughness properties at the same time.
|: Now it is particularly desirable for reinforcing steel

". worth, with high tensile strength not too high

K Yield strength ratio or a large difference

'V. between the tensile strength and the yield point

to have. Should the rod be in ? "■ the area of its plastic deformation above the

Ii reach the yield point, the difference is the

;! Yield point to tensile strength is still a big one

f | Security for the structure given. A statement for

Kj this security is also the value of the elongation,

^ in particular the uniform expansion.

• | Because of the well-known, partly described

(I heat treatment processes it only appears possible

?! increasing the difference between tensile strength and

Ii yield strength when their absolute values decrease

pl or if maintained or increased

p of the absolute values a decrease in the difference

■ ■ received. In contrast to this prior art is

It is the special object of the invention to improve the safety of structural steels || Difference between tensile strength and yield strength

of a low-carbon, cold-worked steel

increase practically while maintaining or increasing

pj hung of the yield point. So one is aiming for

P Increase in tensile strength at the same time more clearly

Increase in elongation.

This object is surprisingly achieved by a method of the type mentioned above, if a steel with 0.06 to 0.20% carbon present in the 10 to 70% cold-worked state is used and the core is at a temperature in the upper crystal recovery range, in particular 450 to 550 ° C, warmed up.

The rapid heating to 800 to 1300 ° C., in particular 900 to 1250 ° C., is preferably carried out.

The process shows particular advantages if the steel is cold-worked by 30 to 50% before the heat treatment will.

Heating is preferably carried out inductively quickly. However, electron beams can also be used for rapid heating can be used. In the case of inductive heating, the choice of frequency and the vote the feed speed compared to the coil dimensions and the energy density the desired Temperature in the bowl. In detail, refer to the prior art cited at the beginning to get expelled.

The quenching of the surface area is preferred except for a temperature below the crystal recovery range, in particular a temperature around 200 ° C. Water has proven to be particularly useful as a detergent, especially water jets.

The application of the method according to the invention to steels with a Carbon content of 0.06 to 0.12% carbon, preferably 0.08 to 0.10% carbon, which is the for

unalloyed steels contain the usual contents of silicon, manganese, etc. or have alloy contents, such as they occur in so-called high-strength structural steels,

The method according to the invention is preferred for reinforcing steel for slack or prestressed reinforcement or for welded wire mesh.

Short-term, i.e. sufficiently rapid heating is important for the method according to the invention so that no significant heat transfer by conduction into the core area of the steel takes place in the heating zone Another part by thermal conduction, according to the known equation of heat conduction, the core region toward transfers the stored in the surface portion (shell) heat content will be sufficient to provide the core to temperatures in the upper crystal recovery area to heat, preferably 4jO to 550 ⁰ C. The temperature in the surface area still remains above Ar> After the heat compensation, accelerated cooling takes place, preferably by means of water. After the surface area has ^{cooled to below 200 °} C., the remaining temperature equalization takes place in air, the quenched surface area easily recovering due to the heat flowing away from the core area

As the following examples show, in a low-carbon, treated according to the invention, During the cold-worked steel, the difference between the yield point and tensile strength is greatly increased Yield strength is maintained or increased. The steel examined was unalloyed and had a carbon content of 0.12% and was with a degree of deformation of 35% to a diameter of 6.5 mm cold formed.

The table shows the technological output values and the final values achieved according to the invention Wärmebehandbngsvt., - come out. As a guide value, the temperature was given in column 1 to which the surface area briefly reached has been heated up.

Warming of the IF OS ob — os strain 4 ") surface ranges to ° C kp / mm ² kp / mm ² kp / mm ² dioin% Starting 60.0 55.0 5.0 8th% material 35% 50 deformation Degree * 00 64.7 56.0 8.7 13% 900 67.7 56.5 11.2 15% 1000 72.0 56.0 16.0 11%

As the table shows, compared to the starting material with 35% degree of deformation after Heat treatment increases strength, yield strength and the difference between the two. With increasing Heating the surface area increases the tensile strength, while the yield point is roughly on the The initial value remains or increases slightly. In all examples, the difference between the increases Tensile strength and the yield point are considerable. Contrary to the known prior art, this leads significant improvement in technological values also leads to a significant increase in elongation.

With the method according to the invention, it is possible, in the case of cold-worked steels, to use, in particular, the To increase the tensile strength, possibly also the yield strength. As the examples show, the elongation is at the same time increased. This will be the difference between the tensile strength and the yield point increased, and it arises in particular for the purpose of use as reinforcing steel in any form - as slack or pre-tensioned reinforcement or in welded wire mesh - a safety distance between the Tensile strength and the yield point,

Claims (7)

Patent claims: 1. Continuous heat treatment process on rod-shaped structural steels to improve the mechanical material properties, such as tensile strength, at which a low-carbon steel in the shell is quickly heated to a temperature above Ac3 that introduced into the shell Heat content warms the core to a temperature below Aci, and the shell is still from a temperature above An is quenched, characterized in that with a significant increase in elongation at the same time, a 10 steel with 0.06 to 0.20% carbon present in the cold-worked state up to 70% is used and the core to a temperature in the upper crystal recovery range, in particular 450 to 550 ° C, heated 2. The method according to claim 1, characterized in that the rapid heating to 800 to 1300 ⁰ C, in particular 900 to 1250 ⁰ C, is carried out. 3. The method according to any one of claims 1 and 2, characterized in that the steel before Heat treatment is cold worked 30 to 50%. 4. The method according to any one of claims 1 to 3, characterized in that inductively fast is heated. 5. The method according to any one of claims 1 to 4, characterized in that the surface area up to a temperature below Crystal recovery area, preferably to a temperature around 200 "C, is quenched. 6. Application of the method according to one of claims 1 to 5 on steels with 0.06 to 0.12% Carbon, in particular 0.08 to 0.10% carbon, which is usual for unalloyed steels Contains silicon, manganese, etc. or alloy contents, such as those in so-called high-strength Structural steels occur. 7. Application of the method according to one of claims I to 6 on reinforcing steels for slack or pre-stressed reinforcement or for welded wire mesh.