The present disclosure relates to a method for producing a high strength steel sheet having improved strength, ductility and formability and to the sheets obtained with the method.
To manufacture various equipment such as parts of body structural members and body panels for automotive vehicles, it is usual to use sheets made of DP (dual phase) steels or TRIP (transformation induced plasticity) steels.
For example, such steels which include a martensitic structure and/or some retained austenite and which contains about 0.2% of C, about 2% of Mn, about 1.7% of Si have a yield strength of about 750 MPa, a tensile strength of about 980 MPa, a total elongation of more than 8%. These sheets are produced on continuous annealing line by quenching from an annealing temperature higher than Ac3 transformation point, down to a quench temperature lower than Ms transformation point followed by heating to an overaging temperature above the Ms point and maintaining the sheet at the temperature for a given time. Then the sheet is cooled down to the room temperature.
Due to the wish to reduce the weight of the automotive in order to improve their fuel efficiency in view of the global environmental conservation it is desirable to have sheets having improved yield and tensile strength. But such sheets must also have a good ductility and a good formability and more specifically a good stretch flangeability.
In this respect, it is desirable to have sheets having a yield strength YS of at least 850 MPa, a tensile strength TS of about 1180 MPa, a total elongation of at least 13% or preferably at least 14% and a hole expansion ratio HER according to the ISO standard 16630:2009 of more than 30% or even 50%. Regarding the hole expansion ratio it must be emphasized that, due to differences in the methods of measure, the values of hole expansion ration HER according to the ISO standard are very different and not comparable to the values of the hole expansion ratio λ according to the JFS T 1001 (Japan Iron and Steel Federation standard).
Therefore, a purpose of the present disclosure is to provide such sheet and a method to produce it.
For this purpose, the present disclosure relates to a method for producing a high strength steel sheet having an improved strength and an improved formability, the sheet having a yield strength YS of at least 850 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 13% and a hole expansion ratio HER of at least 30%, by heat treating a steel sheet whose chemical composition of the steel contains, in weight %:
the remainder being Fe and unavoidable impurities. The sheet is annealed at an annealing temperature TA higher than 865° C. but less than 1000° C. for a time of more than 30 s. Then, the sheet is quenched by cooling down to a quenching temperature QT between 275° C. and 375° C., at a cooling speed of at least 30° C./s in order to have, just after quenching, a structure consisting of austenite and at least 50% of martensite, the austenite content being such that the final structure i.e. after treatment and cooling to the room temperature, can contain between 3 and 15% of residual austenite and between 85% and 97% of the sum of martensite and bainite without ferrite. Then, the sheet is heated up to a partitioning temperature PT between 370° C. and 470° C. and maintained at this temperature for a partitioning time Pt between 50 s and 150 s. Then the sheet is cooled down to the room temperature.
Preferably, the chemical composition of the steel is such that Al≤0.05%.
Preferably, the quenching temperature QT is comprised between 310° C. and 375° C., in particular between 310 and 340° C.
Preferably, the method further comprises, after the sheet is quenched to the quenching temperature QT and before heating the sheet up to the partitioning temperature PT, a step of holding the sheet at the quenching temperature for a holding time comprised between 2 s and 8 s, preferably between 3 s and 7 s.
The present disclosure relates also to a steel sheet whose chemical composition contains in weight %:
The structure of the steel comprises between 3 and 15% of residual austenite and between 85% and 97% of the sum of martensite and bainite, without ferrite.
Preferably, the chemical composition of the steel is such that Al<0.05%.
Preferably, the average grain size of the retained austenite is of 5 μm or less.
The average size of the grains or blocks of martensite and bainite is preferably of 10 μm or less.
The present disclosure will now be described in details but without introducing limitations and illustrated by
According to the present disclosure, the sheet is obtained by hot rolling and optionally cold rolling of a semi product made of a steel which chemical composition contains, in weight %:
The remainder is iron and residual elements resulting from the steelmaking. In this respect, Ni, Cr, Cu, V, B, S, P and N at least are considered as residual elements which are unavoidable impurities. Therefore, their contents are less than 0.05% for Ni, 0.10% for Cr, 0.03% for Cu, 0.007% for V, 0.0010% for B, 0.005% for S, 0.02% for P and 0.010% for N.
The sheet is prepared by hot rolling and optionally cold rolling according to the methods known by those who are skilled in the art.
After rolling the sheets are pickled or cleaned then heat treated.
The heat treatment which is made preferably on a continuous annealing line comprises the steps of:
With such treatment, sheets having a yield strength YS of at least 850 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 13% and a hole expansion ratio HER according to the ISO standard 16630:2009 of at least 30%, or even 50%, can be obtained.
This treatment allows obtaining a final structure i.e. after partitioning and cooling to the room temperature, containing between 3 and 15% of residual austenite and between 85 and 97% of the sum of martensite and bainite without ferrite.
Moreover, the average austenitic grain size is preferably of 5 μm or less, and the average size of the blocks of bainite or martensite is preferably of 10 μm or less.
As an example a sheet of 1.2 mm in thickness having the following composition:
C=0.18%, Si=1.55% Mn=2.02%, Nb=0.02%, Mo=0.15%, Al=0.05%, N=0.06%, the remainder being Fe and impurities, was manufactured by hot and cold rolling. The theoretical Ms transformation point of this steel is 386° C. and the Ac3 point is 849° C.
Samples of the sheet were heat treated by annealing, quenching and partitioning, and the mechanical properties were measured. The sheets were held at the quenching temperature for about 3 s.
The conditions of treatment and the obtained properties are reported at table I.
In this table, TA is the annealing temperature, QT the quenching temperature, PT the partitioning temperature, Pt the partitioning time, YS the yield strength, TS the tensile strength, TE the total elongation, HER the hole expansion ratio according to the ISO standard, RA the proportion of retained austenite in the final structure, RA grain size is the average austenite grain size, M+B is the proportion of bainite and martensite in the final structure and M+B grain size is the average size of the grains or blocks of martensite and bainite.
Example 1, whose structure is shown at
Examples 3 and 4 which are related to the prior art with a quenching temperature higher than Ms, i.e. the structure not being martensitic, show that it is not possible to reach simultaneously the targeted yield strength, total elongation and hole expansion ratio.
Example 5 further shows that with a quenching temperature of 340° C., a partitioning at 470° C. with a partitioning time of 50 s, the sheet has a yield strength higher than 850 MPa, a tensile strength higher than 1100 MPa, a total elongation of about 14% higher than 13% and a hole expansion ratio measured according to ISO standard 16630: 2009 higher than 30%.
Example 6 shows that when the partitioning temperature is too high, i.e. above 470° C., a tensile strength of at least 1180 MPa and a total elongation of at least 13% are not obtained.
Number | Date | Country | Kind |
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PCT/IB2014/002296 | Jul 2014 | IB | international |
This is a divisional of U.S. application Ser. No. 15/322,712 which has a filing date of Dec. 28, 2016 and which is a national stage of PCT/IB2015/055037 filed Jul. 3, 2015 which claims priority to PCT/IB2014/002296 filed Jul. 3, 2014, the entire disclosures of which are hereby incorporated by reference herein.
Number | Date | Country | |
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Parent | 15322712 | Dec 2016 | US |
Child | 17835347 | US |