US 2007/0175548 A1Published application

Method and installation for the production of hot-rolled strip having a dual-phase structure

OwnerKarl-Ernst Hensger, Wolfgang Hennig +2inventors

InventorsKarl-Ernst Hensger, Wolfgang Hennig, Tillmann Bocher, Christian Bilgen

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Abstract

The aim of the invention is to be able to produce dual-phase steels under local conditions even in the existing cooling section of a continuous casting and rolling plant by means of controlled cooling of the hot-rolled strip in two cooling stages following the forming process. Said aim is achieved by respecting the chemical composition of the initial steel within precisely defined limits and cooling in two stages from a finished rolled strip temperature Tfinish of A3−100 K<Tfinish<A3−50 K to a coiling strip temperature Tcoiling of <300° C. (<initial martensite temperature). the cooling speed V1,2 in both cooling stages ranging between 30 and 150 K/s, preferably between 50 and 90 K/s. The first cooling stage is carried out until the cooling curve enters the ferrite range, whereupon the heat released by the transformation of the austenite into ferrite is used for isothermally holding the obtained strip temperature Tconst during a holding time of =5 s until the beginning of the second cooling stage.

Claims4

Dependent claims

Method for producing hot-rolled strip (10) with a dual-phase microstructure consisting of ferrite and martensite, wherein at least 70% of the austenite is transformed to ferrite from the hot-rolled state by a controlled two-stage cooling operation after the finish rolling to a strip temperature below the martensite start temperature in a cooling line (1, 1′) that consists of successive, spaced water cooling units (31-7, 4), wherein, to obtain a hot-rolled strip (10) with a dual-phase microstructure consisting of 70-95% ferrite and 30-5% martensite with high mechanical strength and high formability (tensile strength greater than 600 MPa, elongation after fracture at least 25%) in the cooling line of a continuous casting and rolling installation, starting from a steel with the following chemical composition:

0.01-0.08% C, 0.9% Si, 0.5-1.6% Mn, 1.2% Al, 0.3-1.2% Cr, remainder Fe and customary trace elements:
(a) the two-stage controlled cooling is carried out from a finish rolling strip temperature Tfinish, such that A3=100 K<Tfinish<A3=50 K, to a coiling strip temperature Tcoiling<300° C. (<martensite start temperature), wherein the cooling rate V1,2 in both cooling stages is V=30-150 K/s, and preferably V=50-90 K/s, and
(b) the first cooling stage is carried out until the cooling curve enters the ferrite range, and then the heat of transformation liberated by the transformation of the austenite to ferrite is used for isothermally holding the strip temperature thereby reached Tconst. for a holding time of 5 s until the beginning of the second cooling stage.

Dependencies are inferred from the claim text.