Method for producing hot strip with a multiphase structure

Abstract

For the production of hot strip referred to as TRIP steel (transformation induced plasticity), with a multiphase structure and with outstandingly good deformation properties along with high strengths, from the hot-rolled state, the invention proposes a method which is carried out with a predetermined chemical composition of the steel grade used within the limits 0.12 - 0.25% C; 0.05 - 1.8% Si; 1.0 -2.0% Mn; the remainder Fe and customary accompanying elements and with a combined rolling and cooling strategy in such a way that a structure comprising 40 - 70% ferrite, 15 - 45% bainite and 5 - 20% residual austenite is obtained, wherein the finish rolling of the hot strip (7) is performed to set a very fine austenite grain (d < 8 [mu]m) in the final forming operation (6') at temperatures between 770 and 830 DEG C just above Ar3 in the region of the metastable austenite, and a controlled two-stage cooling (10, 11, 12) is carried out after the last rolling stand (6') of the hot strip (7) to a strip temperature in the range of bainite formation of 320 - 480 DEG C, with a holding time of about 650 - 730 DEG C,; the beginning of which is determined by the entry of the cooling curve into the ferrite region and the duration of which is determined by the transformation of the austenite into at least 40% ferrite.

Description

technical field [0001] The present invention relates to the manufacture of a so-called TRIP steel (Transformation Induced Plasticity Steel) from the hot rolled state by a controlled cooling process behind the final rolling stand, having a high strength at the same time as extremely A method for hot-rolled strip with a multiphase structure with good deformability. Background technique [0002] In TRIP steels, the adjustment of the structure is very complex, since in addition to ferrite and bainite there is also a third phase as retained austenite or, after subsequent deformation, as martensite third phase. Most TRIP steels are currently manufactured in two-stage annealing cycles. The raw material is a hot-rolled or cold-rolled strip in which about 50% α-50% γ-original structure is adjusted by an annealing treatment in an inner critical phase space. Due to the higher solubility of austenite for carbon, there is a higher carbon concentration there. After the annealing treat...

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Problems solved by technology

[0004] Most of the described process controls currently used in the industry for the manufacture of TRIP steel are very cumbersome and costly due to the additional annealing and cooling treatments after the rolling process. The industrial production equipment made of steel materials is used to manufacture these TRIP steels

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