Hot rolled and heat-treated steel sheet and method of manufacturing the same
A hot rolled and heat-treated steel sheet, made of a steel having a composition including, by weight percent: C: 0.03-0.18%, Mn: 6.0-11.0%, Mo: 0.05-0.5%, B: 0.0005-0.005%, S≤0.010%, P≤0.020%, N≤0.008%, and including optionally one or more of the following elements, in weight percentage: Al<3%, Si≤1.20%, Ti≤0.050%, Nb≤0.050%, Cr≤0.5%, V≤0.2%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, the steel sheet having a microstructure including, in surface fraction, from 10% to 60% of retained austenite, from 40% to 90% of ferrite, less than 5% of martensite, carbides below 0.8%, and an inhomogeneous repartition of manganese, characterized by a manganese distribution with a slope above or equal to −40.
1 . A hot rolled and heat-treated steel sheet, made of a steel having a composition comprising, by weight percent:
C: 0.03-0.18%
Mn: 6.0-11.0%
Mo: 0.05-0.5%
B: 0.0005-0.005%
S≤0.010%
P≤0.020%
N≤0.008%
and optionally one or more of the following elements:
Al<3%
Si≤1.20%
Ti≤0.050%
Nb≤0.050%
Cr≤0.5%
a remainder of the composition being iron and unavoidable impurities resulting from processing,
said steel sheet having a microstructure comprising, in surface fraction, from 10% to 60% of retained austenite, from 40% to 90% of ferrite, less than 5% of martensite, carbides below 0.8%, and an inhomogeneous repartition of manganese, defined by a manganese distribution with a slope above or equal to −40.
2 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the steel sheet includes 0.6% or less of carbides.
3 . The hot rolled and heat-treated steel sheet as recited in claim 2 wherein the steel sheet includes 0.5% or less of carbides.
4 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the carbon content is from 0.05% to 0.15%.
5 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the manganese content is from 6.0% to 9.0%.
6 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the aluminium content is from 0.2% to 2.5%.
7 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the Charpy impact energy at 20° C. is higher than 0.4 J/mm 2 .
8 . The hot rolled and heat-treated steel sheet as recited in claim 7 wherein the microstructure includes from 30% to 60% of retained austenite.
9 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the LME index is 0.36 or less.
10 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the steel has a carbon equivalent Ceq lower than 0.4, the carbon equivalent being defined as
Ceq=C %+Si %/55+Cr %/20+Mn %/19−Al %/18+2.2P %−3.24B %−0.133×Mn %×Mo % with elements being expressed by weight percent.
11 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the microstructure includes 0% martensite.
12 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the microstructure includes carbides.
13 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the microstructure includes from 26% to 60% of retained austenite.
14 . The hot rolled and heat-treated steel sheet as recited in claim 1 wherein the microstructure includes from 30% to 60% of retained austenite.
15 . A resistance spot weld of two steel parts made of the hot rolled and heat-treated steel sheet as recited in claim 1 , the resistance spot weld having an α value of at least 30 daN/mm 2 .