DE1583996B2 - HEAT TREATMENT PROCESS TO ACHIEVE IMPROVED UNIFORM MECHANICAL PROPERTIES AND GOOD FORMABILITY ON SHEET METAL WIRE TUBES OR DGL MADE OF STEEL WITH 0.03 TO 0.1% CARBON - Google Patents

Description

The invention relates to a heat treatment method for achieving improved, uniform mechanical properties Properties and good deformability on sheet metal, wires, tubes or the like made of steel with 0.03 to 0.1% carbon, which is one above the ACg point but below the melting point Temperature heated and at a predetermined rate to one below the temperature conversion purikt lying temperature is cooled.

After rolling down a steel body to the required sheet thickness in a corresponding number of hot or cold rolling processes, it is customary to adjust sheet steel to different strength properties, degrees of hardness or softness and different deformabilities by subjecting the sheet or other products to a final heat treatment in annealing furnaces will. It is also known that the hardness and deformability of the finished product depend crucially on the carbon content, on the type and duration of the annealing treatment and, in particular, on the cooling rate. For low-carbon steel sheets, which are to be heated and cooled in a continuous process, a slow cooling for a period of several minutes has been provided in order to achieve a low degree of hardness. Since the hardness that can be achieved here is significantly higher than the values that can be achieved by annealing and cooling stacked sheets in box or hood annealing furnaces, a known proposal (German Auslegeschrift 1192 678) has provided for the production of soft steel sheets with a low carbon content , which are heated for annealing in the continuous process to a _{temperature above the Ac 3} point but below the melting point and then cooled to carry out the cooling so that the sheets in a first cooling stage initially down to a temperature between 550 and 400 ° C at a rate of 300 to 400 ° C / min. cooled, then held for about 2 to 7 minutes at this temperature and then down to room temperature at a rate of a maximum of 100 ° C / min. be cooled down. Nevertheless, this cooling measured in minutes compared to the cooling treatment that is provided in the heat treatment method according to the present invention, a very slow cooling, the result of which in terms of hardness, strength and ductility of the product both from the carbon content of the low-carbon steel and from is heavily dependent on its alloying additives. The carbides content of the steel sheet to be subjected to the annealing process, their conversion into a spherical shape, the grain growth of the structure and the recrystallization, which leads to the fact that "the carbon in solution is deposited on existing carbides during cooling, also play an important role .

Other heat treatments have also been developed in order to obtain sheet metal with improved mechanical properties. A quick annealing treatment at an acceptable temperature ^; can include rapid quenching from the recrystallization temperature to an aging temperature at which the approximately 0.02% supersaturated carbon in the ferrite is precipitated in a stiffening form (quenching aging). So z. B. a steel with a carbon content of 0.06% after recrystallization, quenching and aging at 50 ° C a tensile strength of about 70 kg / mm ² . However, this property is unstable and greatly decreases if the sheet is subjected to temperatures in its subsequent use which are higher than the original aging temperature of 50 ° C, e.g. B. when heating tin cans in an autoclave.

Another known heat treatment that increases tensile strength consists in heating alloy steel sheet, which can have a carbon content of 0.3%, to temperatures between the transformation points Ac ₁ and Ac ₃

lying, and a subsequent strong quenching to create a structure with martensite points in a ferritic structure (U.S. Patent 2,776,230). Here, however, make the difficulty influencing the size and distribution

ao the martensite points in the ferritic structure, this method from the standpoint of producing a well Processable material of uniform and constant properties is impractical. In in practice arise from the normal variation in the size and distribution of the carbide phase in the originally hot-rolled material and by the difficulty of keeping the temperature accurate to which the material can be heated, strong changes in the structure and consequently also in the material properties.

For thinner sheet thicknesses, a martensitic product with a low carbon content can be converted into the austenitic state of the material, e.g. B. above 900 ° C, and strong quenching at a cooling rate of over 2000 ° C / sec. can be obtained. This produces a material that is relatively stable and solid in terms of its properties, the properties of which can be changed by a subsequent brief tempering treatment. The mechanical properties of this martensitic material with a low carbon content after quenching are different depending on the chemical composition, the quenching speed and the austenitic grain size. With an appropriate quenching rate and otherwise the same circumstances, the tensile strength varies between 84 and 126 kg / mm ² depending on the amount of additional components, the most important of which is the carbon content. The deformability or suppleness that results from the longitudinal elongation with axial tension is rather limited and varies between 1 and 2% for a test specimen with a clamping length of 5 cm. Due to the internal diversity of a thick steel blank, the carbon content of the flat-rolled product can vary between 0.05 and 0.10%. This means that a martensitic sheet made from a single ingot can have tensile strength between 84 and 126 kg / mm ² . The rapid annealing of such materials, e.g. B. by a strand annealing treatment to produce a softer material with the increased strength of tempered martensite would allow a broad change in the mechanical properties.

The invention is directed to producing steel with a low carbon content of 0.03 to 0.1% To create sheets, wires, tubes or the like that

have improved, uniform mechanical properties and good deformability, the mechanical properties being essentially determined by a ^{tensile strength of between 38 and 84 kg / mm 2} and a longitudinal extensibility of 12 to 15%. This object is achieved according to the invention in that the cooling of the _{steel heated above the Ac g} point during annealing takes place at a rate of 100 to 500 ^° C. per second. The substantially uniform mechanical properties of the products produced in this way are also independent of the chemical composition of the steel, which is in the normal range for low carbon mild steels, ie steels with a carbon content of 0.03 to 0.10% inclusive obtained from deoxidation treatment or previous thermal and mechanical treatments.

The required cooling rates can be achieved by choosing the appropriate quenching medium for the given heat transfer conditions in terms of strength and Surface quality of the steel, etc. can be secured. So z. B. Cooling Rates from 100 to 500 ° C / sec. for flat-rolled products up to 3 mm thick by cooling in Mineral oil can be obtained at the correct temperature. With the same strength can be used for alloyed Products or products of different sizes and shapes, if different or the same heat transfer conditions exist, the required cooling rates can also be achieved through the use of other quenching media and treatments, including deterrence in slurry, gas jets, regulated gas concentrations, in liquid media, Quenching in sprayed liquids, etc.

The method according to the present invention can also be used both for the treatment of stacked Products as well as for the treatment of continuously promoted products. For example, continuous or semi-continuous continuous furnaces for annealing sheet metal can be used in Sheet or strip form or from black sheet using higher annealing temperatures than usual can be used and the arrangement of a quenching device on the outlet side of the annealing furnace provide in order to achieve the required cooling rates.

The method according to the invention can also be used for finished objects of similar strength which are made from sheet metal or sheet metal strip or by a process in which the strength

ίο the metal is similar to the sheet thickness, e.g. B. at Tubes.

The austenitic transformation point of steel is generally at least 870 ° C and represents is the temperature at which austenite starts to form. This temperature can vary depending on the type and The quality of the alloy materials present in the steel may vary slightly, but it does not go over 900 ° C. In general, it is advisable to heat the steel during annealing to a temperature which is below the melting point, but significantly above the austenitic transformation point, so that the transformation into the austenitic State is going on quickly. Heating temperatures around 1000 ° C are appropriate for this.

The lower limit of the transformation range of steel is the temperature below that during cooling no further structural change takes place.

Mn, P, S, N, Al are suitable as alloy constituents, as are customary in low-carbon mild steels and are also suitable in the customary amounts for products according to the present invention. These alloy additives are preferably 0.0 to 0.5% Mn, 0.0 to 0.05% P, 0.0 to 0.05% S, 0.0 to 0.005% N ₂ , 0.0 to 0.1 % Al.

The invention has the advantage that the steel sheets produced according to this od high strength are significantly softer and more flexible than those used previously for the manufacture of tinplate used sheets that have been affected by double cold rolling, are therefore also easier to process permit. In addition, the invention makes it possible to use steel types of any chemical composition and any thermal or mechanical pretreatment that is easily deformable with high strength Obtain product.

example
Samples of steels with alloy components of grades A to I according to the table below

variety o / o C '/.Mn ° / oP %> S ^{1 of} ₂ VoAl A. 0.088 0.37 0.018 0.030 0.0036 B. 0.046 0.44 0.023 0.027 0.0035 - C. 0.065 0.29 0.018 0.031 0.0035 - D. 0.051 0.32 0.021 0.033 0.0035 - E. 0.045 0.45 0.022 0.029 0.0027 - F. 0.032 0.30 0.024 0.018 0.0027 - G 0.053 0.36 0.018 0.021 0.0038 0.068 H 0.061 0.49 0.024 0.018 0.0028 - I. 0.100 0.57 0.020 0.020 0.0027 -

the effect of the quenching speed on the 65 was brought about by heating to an austenitic structure and then immediately observed in different ab- mechanical properties,
Quenching media quenched to an annealing process. At high quenching speeds the

in continuous promotion and imitate the strength of the chemical composition

There were considerable differences across the range of steel grades examined in tensile strength, as can be seen from FIG. 1 can be seen in the drawing. For those in this diagram The test results shown are the steel grades according to the table above with the Letters ^ to / marked, namely the test samples at 1000 ° C with a Cooling rate of 1100 ° C / sec. deterred.

At increasingly slower cooling rates, the tensile strength decreases, and so does the change the strength is less noticeable, as can be seen from the diagram in FIG. 2 results. In this Trap, the test pieces at 1000 ° C with a cooling rate of 1000 ° C / sec. deterred.

At quenching speeds of 100 to 500 ° C./sec., As occur when quenching thin sheet metal in mineral oils, an essentially Obtain a uniform product, as can be seen from the diagram according to FIG. 3 can be seen. This product has a tensile strength of 56 kg / mm ² with an extensibility of 12 to 15% over a considerable range of the percentage carbon content and despite changes in the other components in the final state. These properties with regard to strength and deformability or softness are particularly satisfactory for sheet metal. When a carbon content of 0.1% is reached, these properties become unstable, and when the percentage carbon content is higher, the mechanical properties are no longer essentially constant. In the experiments that lead to the diagram according to FIG. 3, test samples were made by heating to 1000 ° C at a cooling rate of 300 ° C / sec. Quenched in mineral oil at 20 ° C.

In the diagrams shown, each individual point corresponds to a particular test result. Thus, in the experiments that were related to the daigram according to FIG. 1 carried out four tests for steel types D and E , three tests each for steel types C and G and two tests each for the remaining steel types.

The method according to the invention, which essentially involves heating to form an austenitic Structure and a subsequent, controlled quenching is based on soft sheet metal Obtaining higher strength should not only be used to manufacture tinplate. To The tests carried out can achieve cooling rates of up to 3000 ° C / sec. for sheet thicknesses of about 1 mm with suitable quenching media and quenching conditions. For the Treatment of sheet metal up to at least 2.5 mm thick by quenching up to the specified range There are therefore no difficulties in the critical range of quenching speeds.

Claims (3)

Patent claims: 1. Heat treatment process to achieve improved uniform mechanical properties as well as good deformability of sheets, wires, tubes or the like made of steel with 0.03 to 0.1% carbon, with one above the Ac ₃ point, but below the melting point Hanging temperature is heated and cooled at a predetermined rate to a temperature below the temperature transition point, characterized in that the cooling takes place at a rate of 100 to 500 ° C per second. 2. Heat treatment method according to claim 1, characterized in that the steel heated to a temperature of 1000 ° C and then at a rate of 300 ° C per Second. 3. The method according to claims 1 and 2, characterized in that the cooling in Mineral oil takes place. 1 sheet of drawings