Method for manufacturing a high strength steel plate and high strength steel plate

Abstract

The present disclosure relates to a method for manufacturing a high strength steel plate, comprising the step of providing a hot-rolled steel plate, the step of heating the steel plate to an austenitisation temperature range, the step of quenching the steel plate in two stages with a different cooling speed applied to each stage, the cooling speed of the first stage being higher than the cooling speed of the second stage, the first stage of the two-stage quenching step ending at a temperature range above Ms and the second stage of the quenching step starting at a temperature range above Ms and ending at a temperature range between Ms and Mf, the step of partitioning the steel plate. The disclosure also relates to a high strength steel plate manufactured with the method.

1 . A quenching and partitioning method for manufacturing a high strength steel plate, comprising the steps of:
 providing a hot-rolled steel plate;   heating the hot-rolled steel plate to an austenitisation temperature range;   quenching the hot-rolled steel plate in two stages with a different cooling speed applied to each stage, a cooling speed of a first stage being higher than a cooling speed of a second stage the first stage of the two-stage quenching step ending at a temperature range above Ms and the second stage of the quenching step starting at a temperature range above Ms and ending at a temperature range between Ms and Mf; and   partitioning the hot-rolled steel plate.   
     
     
         2 . The method of  claim 1 , wherein the first stage is a water quenching stage. 
     
     
         3 . The method of  claim 1 , wherein the cooling speed of the first stage is between 10° C./s and 30° C./s, preferably between 12° C./s and 25° C./s, wherein at 15° C./s, a cooling speed corresponding to a mean cooling speed is obtained between an end of the heating step and an end of the quenching step, the mean cooling speed being equal to (T1−T2)/(d/S) with:
 T1: Temperature at a surface of the hot-rolled steel plate at the end of the heating step measured by a pyrometer measuring said temperature at an end of an austenitising furnace where the heating step occurs, 
 T2: Temperature at the surface of the hot-rolled steel plate at the end of the quenching step measured by a pyrometer measuring said temperature at 4 meters after a last cooling section in a quenching unit where the quenching step occurs, 
 d: Distance between a location of T1 measurement and a location of T2 measurement, and 
 S: Steel plate speed between the location of T1 measurement and the location of T2 measurement. 
 
     
     
         4 . The method of  claim 1 , wherein a temperature reached at an end of the first stage is between 350° C. and 650° C., the temperature being measured on a surface of the hot-rolled steel plate. 
     
     
         5 . The method of  claim 1 , wherein the second stage of the two-stage quenching step is an air or water quenching stage. 
     
     
         6 . The method of  claim 1 , wherein a temperature reached at an end of the second stage is between 210° C. and 320° C., preferably between 210° C. and 300° C., the temperature being measured on a surface of the hot-rolled steel plate. 
     
     
         7 . The method of  claim 1 , wherein a duration of the second stage is between 5 and 16 minutes. 
     
     
         8 . The method of  claim 1 , wherein the partitioning step comprises a tempering step of heating the hot-rolled steel plate to a temperature between 300° C. and 500° C., preferably at 400° C., followed by a holding step to partition carbon from martensite to retained austenite, during a time between 15 min and 30 min, preferably 22 minutes, and then a cooling step, by cooling the hot-rolled steel plate in air. 
     
     
         9 . The method of  claim 1 , wherein the high strength steel plate has a thickness between 3 mm and 16 mm. 
     
     
         10 . The method of  claim 1 , wherein the high strength steel plate has a tensile strength of at least 1.300 MPa, a yield strength of at least 800 MPa and a total elongation of at least 11%. 
     
     
         11 . The method of  claim 1 , wherein the high strength steel plate comprises a structure containing retained austenite from 5 to 20% and minimum 65% martensite, at least half of which is tempered martensite, while a sum of ferrite and bainite contents is below 10%. 
     
     
         12 . The method of  claim 1 , wherein the high strength steel plate comprises manganese, in wt %, between 1%-2.6%. 
     
     
         13 . The method of  claim 1 , wherein the high strength steel plate comprises, in wt %, between 0.4%-2.0% of Chromium, between 0.20%-0.8% of Molybdenum, between 0.8%-1.6% of Silicon. 
     
     
         14 . A high strength steel plate manufactured with a quenching and partitioning method, the method comprising:
 providing a hot-rolled steel plate;   heating the hot-rolled steel plate to an austenitisation temperature range;   quenching the hot-rolled steel plate in two stages with a different cooling speed applied to each stage, a cooling speed of a first stage being higher than a cooling speed of a second stage, the first stage of the two-stage quenching step ending at a temperature range above Ms and the second stage of the quenching step starting at a temperature range above Ms and ending at a temperature range between Ms and Mf; and   partitioning the hot-rolled steel plate,   wherein the high strength steel plate has a tensile strength of at least 1.300 MPa, a yield strength of at least 800 MPa and a total elongation of at least 11%.   
     
     
         15 . The high strength steel plate of  claim 14 , wherein the high strength steel plate has a loss of volume of less than 0.450 mm 3  measured via a profilometer via a “pin on disk” method with parameters of a test being
 a load of 20 N; 
 a speed of 20 cm/s; 
 a track radius of 8 mm; 
 a distance of 30.000 cycles; and 
 an alumina ball with a diameter of 6 mm. 
 
     
     
         16 . The high strength steel plate of  claim 14 , wherein the high strength steel plate has a bending performance along transverse and longitudinal directions, corresponding to a ratio of radius to steel plate thickness of less than 3, with a plate thickness of 5 mm,
 the bending tests being performed by bending a 2000 mm×600 mm steel plate having a thickness of 5 mm along the longitudinal direction corresponding to a rolling direction and along the transverse direction corresponding to a direction transverse to the rolling direction, the radius corresponding to the radius of a punch tool performing the bending.   
     
     
         17 . The high strength steel plate of  claim 14 , wherein the high strength steel plate has a bending performance along a transverse direction corresponding to a ratio of radius to steel plate thickness of less than 3 with a plate thickness of 10 mm and having a bending performance along a longitudinal direction corresponding to a ratio of radius to steel plate thickness of less than 4 with a plate thickness of 10 mm,
 bending tests being performed by bending a 2000 mm×600 mm steel plate having a thickness of 10 mm along the longitudinal direction corresponding to a rolling direction and along the transverse direction corresponding to a direction transverse to the rolling direction, the radius corresponding to the radius of a punch tool performing the bending.   
     
     
         18 . The high strength steel plate of  claim 14 , further comprising a structure containing retained austenite from 5 to 20% and minimum 65% martensite, at least half of which is tempered martensite, while a sum of ferrite and bainite contents is below 10%. 
     
     
         19 . The high strength steel plate of  claim 14 , further comprising manganese, in wt %, between 1%-2.6% and between 0.4%-2.0% of Chromium, between 0.20%-0.8% of Molybdenum, between 0.8%-1.6% of Silicon. 
     
     
         20 . (canceled) 
     
     
         21 . The high strength steel plate of  claim 14 , wherein the high strength steel plate has a thickness between 3 mm and 16 mm.