Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced

Patent metadata

Field	Value
Publication number	US-2016355900-A1
Application number	US-201615243610-A
Country	US
Kind code	A1
Filing date	Aug 22, 2016
Priority date	May 11, 2007
Publication date	Dec 8, 2016
Grant date	—

Abstract

The present invention provides a cold-rolled and annealed steel sheet with a strength greater than 1200 MPa, the composition of which includes, the contents being expressed by weight: 0.10%≦C≦0.25%, 1%≦Mn≦3%, A≧0.010%, Si≦2.990%, S≦0.015%, P≦0.1%, N≦0.008%, it being understood that 1%≦Si+Al≦3%, it being understood that Cr+3Mo≧0.3%, Ti in an amount such that Ti/N≧4 and Ti≦0.040%. A balance of the composition includes iron and inevitable impurities resulting from the smelting. The microstructure of the steel includes 15 to 90% bainite, the remainder includes martensite and residual austenite.

First claim

What is claimed is:

1 . A cold-rolled and annealed steel sheet with a strength greater than 1200 MPa, the composition the composition of which comprises, the contents being expresses by weight:
0.10%≦C≦0.25% 1%≦Mn≦3% Al≧0.010% Si≦2.990% S≦0.015% P≦0.1% and N≦0.008%, it being understood that 1%≦Si+Al≦3%, the composition optionally comprising: 0.05%≦V≦0.15% B≦0.005% Mo≦0.25% Cr≦1.65%, it being understood that Cr+3Mo≧0.3%, and Ti in an amount so that Ti/N≧4 and Ti≦0.040%,
the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of said steel comprising 15 to 90% bainite, the remainder consisting of islands of martensite and residual austenite.

2 . The steel sheet as claimed in claim 1 , with an elongation at break greater than 10%, wherein:
Mo<0.005% Cr<0.005% and B=0%,
the microstructure of said steel comprising 65 to 90% bainite, the remainder consisting of islands of martensite and residual austenite.

3 . The steel sheet as claimed in claim 1 , wherein it includes:
Mo≦0.25% and Cr≦1.65%, it being understood that Cr+3Mo≧0.3%, and B=0%,
the microstructure of said steel comprising 65 to 90% bainite, the remainder consisting of islands of martensite and residual austenite.

4 . The steel sheet as claimed in claim 1 , with a strength greater than 1400 MPa and an elongation at break greater than 8%, wherein it includes:
Mo≦0.25% and Cr≦1.65%, it being understood that Cr+3Mo≧0.3%,
the microstructure of said steel comprising 45 to 65% bainite, the remainder consisting of islands of martensite and residual austenite.

5 . The steel sheet as claimed in claim 1 , with a strength greater than 1600 MPa and an elongation at break greater than 8%, wherein it includes:
Mo≦0.25% and Cr≦1.65%, it being understood that Cr+3Mo≧0.3%, the microstructure of said steel comprising 15 to 45% bainite, the remainder consisting of martensite and residual austenite.

6 . The steel sheet as claimed in claim 1 , characterized ion that the composition of said steel includes, the content being expressed by weight:
0.19%≦C≦0.23%.

7 . The steel sheet as claimed in claim 1 , characterized ion that the composition of said steel includes, the content being expressed by weight:
1.5%≦Mn≦2.5%.

8 . The steel sheet as claimed in claim 1 , characterized ion that the composition of said steel includes, the content being expressed by weight:
1.2%≦Si≦1.8%.

9 . The steel sheet as claimed in claim 1 , characterized ion that the composition of said steel includes, the content being expressed by weight:
1.2%≦Al≦1.8%.

10 . The steel sheet as claimed in claim 1 , characterized in that the composition of said steel includes, the contents being expressed by weight:
0.05%≦V≦0.15% 0.0045≦N≦0.008%.

11 . The steel sheet as claimed in claim 1 , characterized in that the composition of said steel includes, the contents being expressed by weight: 0.12%≦V≦0.15%.

12 . The steel sheet as claimed in claim 1 , characterized in that the composition of said steel includes, the content being expressed by weight: 0.00055≦B≦0.003%.

13 . The steel sheet as claimed in claim 1 , characterized in that the the average size of said islands of martensite and residual austenite is less than 1 micron, and the average distance between said islands is less than 6 microns.

14 . A process for manufacturing a cold-rolled steel sheet with a strength greater than 1200 MPa and an elongation at break greater than 10%, in which:
a steel is provided having a composition as claimed in claim 2 ; then a semifinished product is cast from this steel; then said semifinished product is brought to a temperature greater than 1150° C.; then said semifinished product is hot-rolled so as to obtain a hot-rolled sheet; then said sheet is coiled; then said hot-rolled sheet is pickled; then said sheet is cold-rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet; and then said cold-rolled sheet is reheated at a rate V c between 5 and 15° C./s up to a temperature T 1 between Ac3 and Ac3+20° C., and held there for a time t 1 between 50 and 150 s, then said sheet is cooled at a rate V R1 greater than 40° C./s but below 100° C./s down to a temperature T 2 between (M s −30° C. and M s +30° C.), said sheet is maintained at said temperature T 2 for a time t 2 between 150 and 350 s and then it is cooled at a rate V R2 of less than 30° C./s down to the ambient temperature.

15 . A process for manufacturing a cold-rolled steel sheet with a strength greater than 1200 MPa and an elongation at break greater than 8%, in which:
a steel is provided having a composition as claimed in claim 1 , the Mo and Cr contents being Mo≦0.25% and Cr≦1.65%, it being understood that: Cr+3Mo≧0.3%; then a semifinished product is cast from this steel; then said semifinished product is brought to a temperature greater than 1150° C.; then said semifinished product is hot-rolled so as to obtain a hot-rolled sheet; then said sheet is coiled; then said hot-rolled sheet is pickled; then said sheet is cold-rolled with a reduction ratio of between 30 and 80% to obtain a cold-rolled sheet; and then said cold-rolled sheet is reheated at a rate V c between 5 and 15° C./s up to a temperature T 1 between Ac3 and Ac3+20° C., and held there for a time t 1 between 50 and 150 s, then said sheet is cooled at a rate V R1 greater than 25° C./s but below 100° C./s down to a temperature T 2 between B s and (M s −20° C.), said sheet is maintained at said temperature T 2 for a time t 2 between 150 and 350 s and then it is cooled at a rate V R2 of less than 30° C./s down to the ambient temperature.

16 . The manufacturing process as claimed in claim 14 , characterized in that the temperature T 1 is between Ac3+10° C. and Ac3+20° C.

17 . The use of a cold-rolled and annealed steel sheet as claimed in claim 1 , or manufactured by process as claimed in claim 14 , for the manufacture of structural parts or reinforcing elements in the automotive field.

Assignees

• Arcelormittal

Inventors

• Gil Otin Javier
• Moulin Antoine

Classifications

• B22D11/001

  of specific alloys · CPC title

• C21D9/52

  for wires; for strips {; for rods of unlimited length} · CPC title

• C23G1/08

  Iron or steel · CPC title

• C21D8/02 Primary

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C21D8/0236

  Cold rolling · CPC title