DE3113822A1 - Process for heat-treating unalloyed or low-alloyed steels having carbon contents of less than 0.25% - Google Patents

Abstract

In a process for heat-treating unalloyed or low-alloyed steels having carbon contents of less than 0.25%, the steel is heated at a high rate up to a temperature above A1 and then, still before the equilibrium state belonging to the given temperature, i.e. the state of homogenisation of the carbon dissolved in the austenite, is cooled within a period shorter than 500 seconds, counted from the start of heating. In this way, strength values of 600 to 1200 N/mm<2> and elongations between 20 and 12% are achieved.

Description

Process for the heat treatment of unalloyed or low-

alloy steels with carbon contents below 0.25% object The invention is a method for the heat treatment of unalloyed or low-alloyed Steels with carbon contents below 0.25%, with the help of which in addition to an increase the strength of the steel relatively good elongation values can be achieved.

One method for reducing energy consumption is known reducing the weight of the components of various devices, e.g. Motor vehicles. However, the prerequisite for such a weight reduction is Use of vc> n materials 1 ((i t- higher strength values, since in this case the small-sized components must withstand the same stress, which were previously absorbed by the larger-sized components.

The use of alloy steels for such purposes is open by hand, as particularly good strength properties can be achieved with these can. At the same time, however, the use of such alloyed steels also increases the cost of the components to a significant extent, on the one hand as a result of Use of the expensive alloying elements, others j ts infolcae der inevitable application of more complex technologists.

For this reason, research has been initiated focusing on the Improve the mechanical properties of the low-alloy low-carbon Low carbon steels which do not contain steels or alloys at all directed.

The research led in several directions. The fastest results were achieved with so-called micro-alloyed steels. These are included in the generally 0.1-0.15% carbon and, in small amounts, niobium, titanium, tantalum, vanadium and other alloys. The strength properties of these steels are extraordinary good, but their practical use is proportionate for several reasons restricts. The micro-alloys require a much more careful one Technology than the conventional alloys, as the effect of in extraordinary Alloys used in a small amount due to the presence of unexpected foreign matter (impurities) easily picked up in the same amount will. Moreover, the alloying elements also mean in the case of mass production still a significant expense. For the reasons mentioned above, the mass production of micro-alloyed steels is slow to establish itself at the moment.

The other possible way under consideration is use of steels hardened by precipitation hardening.

These are low in carbon and contain, for example, a few% manganese Steels whose strength properties under the influence of precipitation hardening will be significantly cheaper. However, it will become more widespread by adhering to the meticulous technologies required for their manufacture associated difficulties and the high cost of the alloying materials used prevented.

Another possibility is to use the so-called "dual" phase "steels. Binding dorarticren material or those used for: (in production Technology are described in US-PS 3 930 907. The essential in the Manufacturing The dual phase "steels consists in the fact that the material is in the between the temperatures A1 and A3 lying area is heated, in which the Ph; I and <q together are in a state of equilibrium, and that afterwards the cooling with such a Speed succeeds that the -phase is converted into martensite-. In this way secures the island-like in the relatively malleable basic matrix of the finished product Martensite present in an amount of about 10-30% contributes the desired strength still justifiable plasticity parameters.

A solution is also known (US Pat. No. 3,378,369) in which the low-carbon, Material obtained after at least 40% cold deformation above temperature A3 is heated and the fully austenitized steel is rapidly cooled.

The completely martensitic steel obtained in this way has good strength properties and relatively good ductility. Of the However, material treated in this way can only be obtained by cold working, whereby the extent of cold deformation is also precisely defined.

The invention deviates from the solutions described Process created that offers the possibility, not only of low-alloy steels, but practically unalloyed low-carbon steels with increased strength properties and justifiable malleability (ductility) with the help of a simpler and less To produce energy-related heat treatment.

Which has a high strength and a relatively good malleability comprising steel can be manufactured according to the invention so that a heat treatment is made, in the course of which the material quickly reaches above the temperature A1 heated and then before reaching the temperature belonging to the given temperature State of equilibrium, i.e. the homogenization of the carbon in austenite, within a period of time calculated from heating that is shorter than 500 seconds is cooled quickly.

In the course of the process according to the invention, generally exceeds the time of heating and holding is not 300 seconds. The cheapest the range between 5-0.1 sec can be used. Heating is preferred by means of the known direct electrical resistance heating, i. e. that the heat is generated by the electrical current carried through the material is used. Of course, the heating can also be done in other ways, e.g.

in a metal or salt bath or an overheated oven.

The basis of the invention is in particular the knowledge that then, when the steel heats up rapidly to the temperature range above the temperature A1 and the partial or complete disintegration of the carbides is permitted becomes without time for achieving a homogeneous distribution of the carbon let, and the steel from this the state of equilibrium is far from corresponding State is rapidly cooled, the structure of the steel is intolquX the inhomogeneous Carbon distribution and rapid cooling changed so that the convert areas rich in carbon to martensite or beeinite and these martensite or beeinite islands are embedded in the ferritic base material. As a result, the strength increases to a value of 600-1200 N / mm2, with adjusts the elongation to a value between 20-12%.

It should be noted that the heat treatment both after a Hot deformation as well as after a cold: deformllng can be made. To The material can undergo plastic, large-scale hot deformation after it has emerged are fed directly into the cooling device between the rollers. In certain In some cases, it is advisable to agglomerate the carbides before the heat treatment bring about in steel.

A fundamental advantage of the method according to the invention is that with its help alloy elements can be saved in significant quantities, the manufacturing technology compared to the solutions described above can be carried out much easier and cheaper.

[) The above and other details of the finding will be based on of exemplary embodiments described.

Example 1 The heat treatment was a 0.08 wt .-% carbon containing steel plate with the help of one of the production of the aforementioned "dual-phase" steels appropriate technology carried out. The thickness of the treated panel was 1.88 mm and the material contained the following components in addition to carbon: silicon 0.03 wt% manganese 0.33 molybdenum 0.011 titanium 0.24 vanadium 0.014 nitrogen 60 ppm Oxygen 140 ppm The thermoformed steel plate was heated to 870 ° C. The common The heating and holding time was 5 minutes. After cooling on In the air, the tensile strength of the material was 470 Newtons / mm2, the elongation 16%.

Example 2 A likewise low-carbon, but containing more manganese Steel was treated with a technology similar to that given in Example 1.

The material composition was as follows: carbon 0.11% by weight Silicon 0.55 manganese 1.45 molybdenum 0.21 aluminum 0.05 niobium 0.04 nitrogen 50 ppm The sheet, hot-rolled to a thickness of 2.5 mm, was heated to a temperature heated to 790 ° C and then cooled in the air. The q common duration of heating and keep warm was 3 minutes. After the treatment, the tensile strength was of steel 750 Newton / mm2, its length 14%.

The steel given in the first example was essentially microalloyed Steel, the second a normal steel alloyed with manganese. These became a conventional one Subjected to heat treatment. In the next examples, the invention is appropriate Heat treatments of steels with similar carbon contents and those achievable with them Results presented.

Example 3 The heat treatment was a carbon containing 0.1% by weight unalloyed steel strip. The thickness of the tape was 1 mm, its width 20 mm. The heat treatment was carried out with direct resistance heating.

In view of the fact that in the course of the heat treatment corresponding to the process according to the invention the material temperature changes constantly, i.e. that the heat treatment essentially consists of rapid heating and rapid cooling, the holding temperature or heat treatment temperature and time taken in the conventional sense cannot be specified will. For this reason, in the present example the duration is characterized by the heating time and a so-called c! IEclctivc temperature is given, which was calculated according to the following ormcl (see Prohászka, Theoretical and practical problems of the acceleration of heat treatment processes, Neue Hütte 1977/6 336-340) : where: Q - the activation energy R - the general gas constant T - the absolute temperature.

The effective temperature of the heat treatment according to the invention was 980 "C, the duration of which is 2 seconds. The tensile strength of the treated material reached 60p Newton / mm2, the elongation 20%.

From this it can be seen that the heat treated according to the invention Steel essentially corresponds to that in Example 1 with the difference that it has none Contains micro-alloys.

At the same time it can also be seen that the result of the heat treatment both the li'estigkeit and the opinion values turn out to be more favorable.

Example 4 The heat treatment was again a 1 mm thick and 20 mm wide band. The carbon content of the material was 0.13% by weight and the material also contained about 1.6% by weight manganese. The value of the carbon content and the presence of manganese accordingly correspond to those in the second example presented steel. The material does not contain any other leaching substances.

The heat treatment was carried out with a heating to 980 "C. The heat treatment time was 2 Seconds. After the heat treatment showed the material.

Tensile strength of 800 Newton / mm² and an elongation of 20i.

It is easy to see that these values also correspond to the The steel treated in the second example exceeded the values noted, though the material did not contain any other alloying substances besides manganese.

Example 5 The material described in the third example was used below a heat treatment at a temperature of 10300C for a period of 2 seconds subjected. Here, a strength value of 1000 Newton / mm2 and an elongation of 15% rretcllt.

Example 6 The fabric presented in Example 4 was subjected to a heat treatment at a temperature of 10300C for a period of 2 seconds. the The tensile strength achieved was 1200 Newton / mm2, the elongation value was 12%.

It can be seen that the strength values increase with the increase in temperature to a certain extent increase, at the same time j e (o (: h the elongation naturally decreases begins.

There have also been attempts to change the duration of the heat treatment performed. In the course of this it was found that the reduction in treatment time the results achieved are essentially unaffected. To carry out However, heat treatments with periods of time shorter than 0.1 seconds are already one pretty high Amperage required so that the heating is extraordinary must be done quickly. For this reason, the invention requires ß (heat treatment box such short heat treatment times already costly Power converter.

If the heat treatment time is increased, the tensile strength values reached begin to go back already without increasing the elongation. This is going through the following two examples demonstrated.

Example 7 The material presented in the third example became a Heat treatment at a temperature of 1 1000 ° C for a period of 150 seconds subjected. After the treatment, the tensile strength of the steel was 550 Newtons / min2, its elongation 17%.

Example 8 The material described in the fourth example became a Subjected to heat treatment at a temperature of 10300C.

The heat treatment time was 450 seconds. The results obtained were as follows: tensile strength: 480 Newtons / mm2, elongation 20%.

From the voryestierter examples it is clear that the inventive Process Possibilities for such a heat treatment of low-carbon, unalloyed or low alloy steels offers:, with the help of which better strength and ductility properties than conventional steels can be produced. At the same time, the heat treatment according to the invention is easier and requiring less energy than most used in similar steels Heat treatments. The heat treatment of having good strength properties and This is because the Tegel contains steels with acceptable ductility properties also a softening operation. This process can be carried out if the Process according to the invention are omitted.

The results obtained also prove that the properties the steels heat treated in this way are at least as good as those of the others Processes are produced without the use of alloying elements.

Claims (7)

Process for the heat treatment of unalloyed or low-alloy steels with carbon contents below 0.25 ° patent claims 1. Process for the heat treatment of unalloyed or low-alloy steels with carbon contents below 0.25%, thereby characterized in that the steel at high speed up to above temperature A1 heated and then before reaching the temperature belonging to the given temperature State of equilibrium, i.e. the homogenization of the dissolved carbon in the Austenite, within 500 seconds from the start of heating to be counted from <jekülii t. 2. The method according to claim 1, characterized (lekennzeichnot, <jaß der Material cooled down within 300 seconds, calculated from the start of heating will. 3. The method according to claim 1 or 2, characterized in that the Material cooled within 5-0.1 seconds from the start of heating will. 4. The method according to any one of claims 1 to 3, characterized in that that the material is heated with direct electrical resistance heating. 5. The method according to any one of claims 1 to 3, characterized in, that the heating is carried out in a metal bath. 6. The method according to any one of claims 1 to 3, characterized in that that the heating is carried out in a salt bath. 7. The method according to any one of claims 1 to 3, characterized in that that the heating is done in an overheated oven.