Method for thread rolling of a steel element

Abstract

Method for thread rolling of a tempered steel element (1), characterized in that the thread forming process is included in the cooling process of a tempering phase of the tempering process, wherein the steel element (1) is thread rolled by means of hot forming below the tempering temperature (Ta) of the tempering process at a temperature level between 200 and 450 °C.

Description

[0001] The invention relates to a method for thread rolling of a tempered steel element, according to the preamble of claim 1.

[0002] It is known to produce screw threads on both heat-treated and non-heat-treated steel elements by cold forming using rotating thread-cutting rollers or die rollers, in order to avoid the material losses, longer production times, and fiber cuts that occur with machining. Alternatively, but far less frequently, the steel elements are heated to forging temperature in their unheat-treated state and then thread-cut, which reduces tool wear but simultaneously eliminates the work hardening effect.

[0003] Furthermore, a method of the type mentioned above is known from DE 11 2010 003 614 T5, in which the thread rolling temperature is changed within wide limits between room temperature and a maximum value below the Ac1 transformation point, depending on the formability of the tempered steel element.

[0004] Document DE 946 629 B relates to a method for preheating steel workpieces intended for thread rolling. To improve the rollability of the workpieces and thus reduce rolling times, document DE 946 629 B proposes preheating workpieces made of tempered steel to below tempering temperatures shortly before the rolling process, and those made of softer materials to temperatures approximately between 20 and 150°C.

[0005] Document DE 26 35 188 A1 relates to a process for manufacturing high-strength screws, threaded components, bolts, axles, and the like from steel wire with a carbon content of approximately 0.3 to 0.7% and a manganese content of < 1% (piece analysis) and a tensile strength of approximately 800 N / mm² and higher. In addition to machining, the process involves heat treatment of the material to influence its microstructure and residual stresses. To reduce the energy required for heat treatment and to avoid environmental concerns and quality reductions, document DE 26 35 188 A1 proposes starting with patented and, if necessary, drawn steel wire, cold-forming this steel wire into blanks, and then tempering these blanks at a temperature of 200 to 400°C. Tempering is carried out before or after threading.

[0006] The object of the invention is to design a method of the type mentioned above in such a way that the dynamic strength of the thread-rolled steel element is significantly increased.

[0007] This problem is solved according to the invention by the method characterized in claim 1.

[0008] According to the invention, by thread rolling of the tempered steel element within selected, relatively narrow temperature limits between 200 and a maximum of 450°C and preferably between 250 and 350°C, a significant increase in crack resistance is unexpectedly achieved, especially in the critical base area of ​​the threads, thereby significantly increasing the dynamic strength of the thread element produced in this way, namely by at least 15%.

[0009] By including the thread forming process in the cooling process of the tempering phase of the tempering process, the heat energy required for tempering is used for hot rolling of the steel element, thus eliminating the need to reheat the steel element after the end of the tempering process.

[0010] The invention will now be explained in more detail with reference to an exemplary embodiment in conjunction with the drawings. These show, in a highly schematic representation: Fig. 1. A temperable steel element in the hardening and tempering phases of the tempering process; and Fig. 2 the tempered steel element during thread rolling in the course of the cooling process of the tempering phase together with the rolling tool in the front ( Fig. 2a) and in side view ( Fig. 2b).

[0011] The in Fig. The round bar 1 shown, made of temperable steel, is heated in a hardening station 2 to a temperature level, usually above 700 °C, which depends on the specific steel grade, up to the austenite region, and then quenched. The now hardened steel element 1 is then held in a tempering station 3 at a temperature Ta between 450 and 650 °C, and even higher for some steel grades, and then slowly cooled.

[0012] The thread rolling of the hardened steel element 1 is included in the tempering phase of the tempering process in such a way that, during the cooling process of the tempering phase, when the temperature of the steel element 1 has fallen below the tempering temperature Ta to a level between 200 and 450°C, in particular between 250 and 350°C and preferably about 300°C, the steel element 1 is placed in a pressure rolling tool 4 ( Fig.2) is thread-formed, which contains several rotating thread rollers 5 that are pressed against the steel element 1 and thereby imprint a thread 6 onto it by means of hot forming.

[0013] As a result of the combined tempering and rolling process, a steel element 1 is produced which, compared to steel elements tempered and thread-rolled in the usual way, has a significantly increased crack resistance due to hot rolling in the specified temperature range, especially at the thread root of the critical thread turns of the thread 6, and consequently at least up to 15% higher dynamic strength.

Claims

[1] Method for thread rolling of a tempered steel element (1), characterized by , that the thread forming process is included in the cooling process of a tempering phase of the tempering process, wherein the steel element (1) is thread rolled by means of hot forming below the tempering temperature (Ta) of the tempering process at a temperature level between 200 and 450 °C. [2] Method according to claim 1, characterized by , that the steel element (1) is thread-rolled at a rolling temperature of a maximum of 400 °C, preferably between 250 and 350 °C, in particular about 300 °C.

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