Process for direct production of a trip steel strip in an integrated casting-rolling plant and a trip steel strip produced by the process

Abstract

A process for production of a TRIP steel strip that includes supplying a finished strip to a first cooling group of a cooling sector to force-cool the core of the finished strip to a second exit temperature (TA 2 ) in the range 620° C. to 700° C., whereby, upon exiting the first cooling group, the core has a predominantly austenitic microstructure, transporting the finished strip to a third cooling group establishing, during the transport, a second cooling rate of the core of the finished strip in the range −25 K/s to 20 K/s to convert a first portion of the austenitic microstructure into a ferritic microstructure, force-cooling the core of the finished strip in the third cooling group to a third exit temperature (TA 3 ) which is not more than the bainite starting temperature (BS) to at least partially convert a second portion of the austenite into a bainitic microstructure.

1 . A process for producing a TRIP steel strip in an integrated casting-rolling plant,
 wherein the integrated casting-rolling plant comprises a finishing mill train and a cooling sector,   wherein the finishing mill train is supplied with a hot precursor strip which is finish-milled into a finished strip by the finishing mill train   wherein after the finish-milling of the finished strip the finished strip is supplied to a first cooling group of the cooling sector and in the first cooling group a core of the finished strip is force-cooled to a second exit temperature (TA 2 ) such that the second exit temperature (TA 2 ) is in a range from 620° C. to 700° C. inclusive,   wherein upon exiting the first cooling group the core of the finished strip has a predominantly, particularly preferably completely, austenitic microstructure,   wherein the finished strip is transported to a third cooling group which is arranged spaced apart from the first cooling group,   wherein during the transport of the finished strip between the first cooling group and the third cooling group a second cooling rate of the core of the finished strip is established,   wherein the second cooling rate of the core of the finished strip is −25 K/s to 20 K/s inclusive and during transport a first portion of the austenitic microstructure is converted into a ferritic microstructure in the finished strip,   wherein in the third cooling group the core of the finished strip is force-cooled to a third exit temperature (TA 3 ) which is not more than the bainite starting temperature (BS) so that a second portion of the austenite of the finished strip is at least partially converted into a bainitic microstructure.   
     
     
         2 . The process as claimed in  claim 1 ,
 wherein after exiting the first cooling group the cooled finished strip is transported to a second cooling group of the cooling sector,   wherein a forced cooling of the finished strip in the second cooling group is deactivated and in the second cooling group the finished strip is transported to a third cooling group of the cooling sector.   
     
     
         3 . The process as claimed in  claim 1 ,
 wherein after passing through the cooling sector the finished strip is coiled into a coil,   wherein the finished strip is cooled from the third exit temperature (TA 3 ) to an ambient temperature (TU) when coiled in the coil,   wherein upon cooling of the coiled finished strip to ambient temperature (TU) a remaining third portion of the austenite of the finished strip is enriched with carbon (C) present in solution to form a metastable austenite phase proportion.   
     
     
         4 . The process as claimed in  claim 1 ,
 wherein in the first cooling group the finished strip is force-cooled such that a first cooling rate of the core of the finished strip is established,   wherein in the third cooling group the finished strip is force-cooled such that a third cooling rate of the core of the finished strip is established,   wherein the second cooling rate is lower than the first cooling rate and/or the third cooling rate,   wherein preferably the first cooling rate and/or the third cooling rate of the core of the finished strip is 20 K/s to 400 K/s inclusive, in particular 50 K/s to 200 K/s inclusive.   
     
     
         5 . The process as claimed in  claim 1 ,
 wherein upon exiting the finishing mill train the core of the finished strip has a first exit temperature (TA 1 ) above a ferrite precipitation temperature (AR 3  temperature), in particular of 800° C. to 950° C., in particular of 830° C. to 860° C.   
     
     
         6 . The process as claimed in  claim 1 ,
 wherein the finished strip is transported from the first cooling group into the third cooling group over a second time interval (t 2 ) of 3 seconds to 8 seconds, in particular of 4 seconds to 5 seconds.   
     
     
         7 . The process as claimed in  claim 1 ,
 wherein the core of the finish-milled finished strip exits the second cooling group at a third exit temperature (TA 3 ) of 580° C. to 680° C. inclusive, in particular of 620° C. to 660° C. inclusive, and is transported into the third cooling group of the cooling sector,   wherein preferably the third exit temperature (TA 3 ) is greater than the austenite-ferrite conversion temperature (Ar 1 ),   wherein upon exiting of the finished strip from the third cooling group the core of the finished strip has a fourth exit temperature (TA 4 ) of 180° C. to 450° C. inclusive, in particular 330° C. to 420° C. inclusive, in particular 360° C. to 390° C. inclusive.   
     
     
         8 . The process as claimed in  claim 1 ,
 wherein the core of the finished strip is cooled from the fourth exit temperature (TA 4 ) to an ambient temperature (TU) over a fourth time interval (t 4 ) of 24 hours to 72 hours.   
     
     
         9 . The process as claimed in  claim 1 ,
 wherein a thickness of the precursor strip upon entry into the finishing mill train is 4 mm to 25 mm, in particular 6 mm to 18 mm,   wherein the finishing mill train reduces the thickness of the precursor strip to that of the finished strip of 0.6 mm to 6 mm, in particular to 0.8 mm to 2 mm.   
     
     
         10 . The process as claimed in  claim 1 ,
 wherein the finished strip has a chemical composition in percent by weight of 0.15% to 0.25%, in particular 0.19% to 0.21%, inclusive of C, 1.0% to 2.0%, in particular 1.4% to 1.6%, of Mn, 1.0% to 1.5%, in particular 1.1% to 1.3%, of Si, 0.3% to 0.7%, in particular 0.45% to 0.55%, inclusive of Al, balance Fe and unavoidable impurities.   
     
     
         11 . The process as claimed in  claim 1 ,
 wherein the integrated casting-rolling plant comprises a continuous casting machine having a mold and a single- or multi-stand roughing mill train,   wherein a metallic melt is cast in the mold to afford a partially solidified thin-slab strand,   wherein the partially solidified thin-slab strand is supported and deflected,   wherein the roughing mill train is directly supplied with the partially solidified thin-slab strand from the continuous casting machine,   wherein the roughing mill train rolls the thin-slab strand into the precursor strip,   wherein the precursor strip is uninterruptedly supplied to the finishing mill train.   
     
     
         12 . The process as claimed in  claim 11 ,
 wherein an intermediate heating means is arranged between the roughing mill train and the finishing mill train,   wherein the intermediate heating means heats a core of the precursor strip by at least 100° C. to 300° C. inclusive, in particular to 1100° C. to 1180° C.,   wherein the heated precursor strip is supplied to the finishing mill train.   
     
     
         13 . A TRIP steel strip, produced by a process as claimed in  claim 1 ,
 having a chemical composition in percent by weight of 0.15% to 0.25%, in particular 0.15% to 0.21%, inclusive of C, 1.0% to 2.0%, in particular 1.4% to 1.6%, of Mn, 1.0% to 1.9%, in particular 1.1% to 1.3%, of Si, 0.3% to 0.7%, in particular 0.45% to 0.55%, inclusive of Al, balance Fe and unavoidable impurities,   wherein at room temperature the finished strip has the following microstructure based on percent by volume: from 40% to 60% inclusive ferrite, in particular from 45% to 55% inclusive ferrite, from 8% to 15% inclusive metastable residual austenite, balance preferably cementite-free bainite,   wherein the TRIP steel strip preferably has a thickness of 0.6 mm to 6 mm inclusive, in particular 0.8 mm to 2 mm inclusive.