Method of production of steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding

Abstract

A method for producing steel sheet semi-finished products (P) by press hardening is provided that utilizes locally-modified structure in spots for welding. During this method, a steel semi-finished product (P) is heated to a temperature at which complete transformation of ferritic-pearlitic structure to austenite occurs. Afterwards, the semi-finished product (P) is cooled locally in spots for welding to a temperature close to the M_f, after which heat conduction from the surroundings causes reheating and consequently annealing of the hardening structure. Subsequently, the semi-finished product (P) is formed into a drawn part which, at the same time, is hardened in the tool, after which the sheet semi-finished product (P) is retrieved from the tool, once the required temperature was reached.

Description

RELATED APPLICATIONS[0001]This application claims the priority benefit of Czech Patent Application Serial No. PV 2019-513 entitled “Method of production of steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding,” filed Aug. 7, 2019, the entire disclosure of which is incorporated herein by reference.FIELD OF THE INVENTION[0002]This invention relates to a method of producing steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding.BACKGROUND ART[0003]Materials for manufacturing steel sheet semi-finished products may include high-strength MnB steels, which possess a martensitic microstructure after processing. Such materials may be useful in car body construction. These materials may be produced by press hardening, which is a method of hardening in a tool. Therefore, these materials may show good strength, but may also exhibit a relatively poor ability to undergo plastic deformation, w...

AI Technical Summary

Benefits of technology

[0010]This method of cooling produces hardened spots in the semi-finished product, whereas the temperature of the remaining material is above the temperature of transformation to hardening structures, martensite and possibly bainite. Cooling is preferably affected by means of nozzles through which pressurized cooling medium is supplied to the spots to be cooled. A shroud guides the medium away from the cooled spot. The shroud also secures that a defined spot is cooled. The intensity of the process can be enhanced by providing cooling on both faces of the sheet blank. In alternative embodiments, a contact-based cooling method may be used, which involves pressing cooled blocks against the semi-finished product surface. Cooling is ended after the spot reaches a temperature at which a hardening structure forms. Intensive cooling dissipates heat energy only from the area to which it is applied, i.e., the spot where a weld is to be made. Cooling also removes the transformation energy released during transformation of austenite to hardening structure.

Problems solved by technology

Therefore, these materials may show good strength, but may also exhibit a relatively poor ability to undergo plastic deformation, which has highly adverse consequences in welded structures, namely spot-welded joints.

The risk of the structure failing under external loading, arising for instance from a car collision, increases with strength.

The drawbacks to this procedure include longer production times and the energy needed for the local annealing operation.

Images