STRENGTH, DUCTILITY AND FORMABILITY

The present invention relates to a method for producing a high strength coated steel sheet having improved strength, ductility and formability and to the sheets obtained with the method.

To manufacture various equipments such as parts of body structural members and body panels for automotive vehicles, it is usual to use coated sheets made of DP (dual phase) steels or TRIP (transformation induced plasticity) steels.

For example, such steels which include a martensitic microstructure and/or some retained austenite and which contain about 0.2% of C, about 2% of Mn, about 1.7% of Si have a yield strength of about 750 Pa, 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 Ac ₃ transformation point, down to an overaging temperature above Ms transformation point and maintaining the sheet at the temperature for a given time. Then the sheet is either hot dip galvanized or electro- galvanized.

To reduce the weight of the automotive so as 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.

In this respect, it is desirable to have sheets having a yield strength YS of at least 550 MPa, a tensile strength TS of about 980 MPa, a uniform elongation of at least 12% and a total elongation of at least 18%. Therefore, the purpose of the present invention is to provide such sheet and a method to produce it.

Therefore, the invention relates to a method for producing a coated steel sheet having a microstructure containing between 5% and 25% of intercritical ferrite, at least 10% of retained austenite and at least 65% of martensite and bainite by heat treating and coating a steel sheet wherein the chemical composition of the steel contains in weight %:

0.15%≤C < 0.25%

1.2%≤ Si < 1.8% 2%≤ Mn≤ 2.4%

0.1% < Cr < 0.25%

Al <0.5%

the balance being Fe and unavoidable impurities, and wherein the heat treatment and coating operations comprise the successive following steps:

Heating and annealing the sheet at an annealing temperature TA between TA1 = Ac3 - 0.45 ^* (Ms - QT) where QT is the quenching temperature between 180°C and 300°C and TA2 = 830°C for a time of more than 30 sec,

- quenching the sheet by cooling it down to the quenching temperature QT

- heating the sheet up to a partitioning temperature PT between 380°C and 480°C for a partitioning time Pt between 10 sec and 300 sec

coating the sheet either by electro-galvanizing or vacuum coating after cooling to the room temperature or by hot dip coating the sheet and then cooling it down to the room temperature

Preferably, the method according to the invention is such that: 0.17% < C < 0.21 %.

In another embodiment, the method according to the invention is such that:

1.3% < Si < 1.6 %.

In another embodiment, the method according to the invention is such that:

2.1 % < Mn < 2.3 %. In a preferred embodiment, the method according to the invention is such that the partitioning temperature PT is between 430°C and 480°C for a partitioning time between 10 s and 90 s.

In another embodiment, the method according to the invention is such that the partitioning temperature PT is between 380°C and 430°C for a partitioning time between 10 s and 300 s.

In a preferred embodiment, the method according to the invention is such that the hot dip coating step is a galvanizing or galvannealing step. In another embodiment, the method according to the invention is such that, hot dip coating step is done using an Al or Al alloyed bath.

The object of the invention also relates to a steel sheet wherein the

0.15%≤C < 0.25%

1.2% < Si < 1.8%

2%≤ Mn≤ 2.4%

0.1≤Cr≤0.25%

Al <0.5%

the balance being Fe and unavoidable impurities, wherein the microstructure comprises of between 5% and 25% of intercritical ferrite, at least 10% of residual austenite and at least 65% of combined martensite and bainite

Preferably, the steel sheet according to the invention is such that: 0.17% < C < 0.21 %.

In another embodiment, the steel sheet according to the invention is such that:

1.3% < Si < 1.6 %.

In another embodiment, the steel sheet according to the invention is such that:

2.1 % < Mn < 2.3 %.

In a preferred embodiment, the steel sheet according to the invention is coated with a Zn or Zn alloy or even with an Al or Al alloy. In a preferred embodiment, the steel sheet according to the invention has a yield strength of at least 550 MPa, a tensile strength of at least 980 MPa, a uniform elongation of at least 12% and a total elongation of at least 18%.

The invention also has as an object the use of a steel sheet or the production method described to make parts for automotive body in white.

The invention will now be described in details but without introducing limitations.

According to the invention, the sheet is obtained by hot rolling and cold rolling of a semi product which chemical composition contains, in weight %: - 0.15 to 0.25% of carbon, and preferably 0.17% to 0.21 %, to ensure a satisfactory strength and improve the stability of the retained austenite. This retained austenite content is necessary to obtain sufficient uniform and total elongations. If carbon content is above 0.25%, the hot rolled sheet is too hard to cold roll and the weldability is insufficient. If carbon content is below 0.15 %, yield and tensile strength levels will not reach respectively 550 and 980 MPa.

- 1.2% to 1.8%, preferably 1.3% to 1.6% of silicon in order to stabilize the austenite, to provide a solid solution strengthening and to delay the formation of carbides during overaging without formation of silicon oxides at the surface of the sheet which is detrimental to coatability.

- 2% to 2.4% and preferably 2.1% to 2.3% of manganese. The minimum is defined to have a sufficient hardenability in order to obtain a microstructure containing at least 65%) of martensite and bainite, tensile strength of more than 980 MPa and the maximum is defined to avoid having segregation issues which are detrimental for the ductility if Mn content is above 2.3%.

-0.1 % to 0.25% of chromium is necessary. At least 0.1% is needed to increase the hardenability and to stabilize the retained austenitic in order to delay the formation of bainite during overaging. A maximum of 0.25% of Cr is allowed, above a saturation effect is noted, and adding Cr is both useless and expensive.

- up to 0.5% of aluminum which is usually added to liquid steel for the purpose of deoxidation. Preferably, the Al content is limited to 0.05 %. If the content of Al is above

0.5%, the austenitizing temperature will be too high to reach during annealing and the steel will become industrially difficult to produce.

The balance is iron and residual elements resulting from the steelmaking. In this respect, Ni, Mo, Cu, Nb, V, Ti, 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, 0.02% for Mo, 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. Nb content is limited to 0.05% and Ti content is limited to 0.05% because above such values large precipitates will form and formability will decrease, making the 18% of total elongation more difficult to reach. The sheet is prepared by hot rolling and cold rolling according to the methods known by those which are skilled in the art.

Optionally, the hot rolled sheet is batch annealed before cold rolling at a temperature TBA in the range 550°C - 650°C for more than 5 hours to ensure a better cold-rollability of the hot rolled sheets.

After rolling the sheets are pickled or cleaned then heat treated and either hot dip coated, electro-coated or vacuum coated. The heat treatment which is made preferably on a combined continuous annealing and hot dip coating line comprising the steps of:

- Annealing the sheet at an annealing temperature TA between TA1 = Ac3 - 0.45*(Ms - QT) and TA2 = 830°C where:

Ac3 = 910 - 203[C] ^{1 2} - 15,2[Ni] + 44,7[Si] + 104[V] + 31 ,5[Mo] + 13,1 [W] - 30[Mn] - 1 1 [Cr] - 20[Cu] + 700[P] + 400[AI] + 20[As] + 400[Ti]

Ms = 539 - 423[C] - 30.4 [Mn] - 17.7 [Ni] - 12.1 [Cr] - 1 1 [Si] - 7.5 [Mo]

QT must be between 180° and 300°C

Chemical composition elements are given in wt %. This is to ensure a maximum fraction of 25% of intercritical ferrite and to ensure a minimum of 5% of intercritical ferrite i.e ferrite formed during an intercritical annealing between approximately 721 °C and Ac3. The sheet is maintained at the annealing temperature i.e. maintained between TA - 5°C and TA + 10°C, for a time sufficient to homogenize the chemical composition and the microstructure. This time is of more than 30 sec but preferably does not need to be of more than 300 sec.

- Quenching the sheet by cooling down to the quenching temperature QT which is between 180°C and 300°C. Such temperature is lower than the Ms transformation point and is reached at a cooling rate enough to avoid polygonal ferrite and bainite formation during cooling. Cr is helpful to avoid such formation. By quenching, it is meant a cooling rate higher than 30°C/s. The quenching temperature is between 180°C and 300°C in order to have, just after quenching a microstructure consisting of intercritical ferrite, martensite and austenite. This microstructure contains between 5% and 25% of intercritical ferrite, at least 50% of martensite and contains balance amount of austenite. - Then the steel is reheated up to a partitioning temperature PT between 380°C and 480°C and preferably between 430°C and 480°C if the sheet is hot dip coated. For example, the partitioning temperature can be equal to the temperature at which the sheet must be heated in order to be hot dip coated, i.e. between 455°C and 465°C. On the other hand, the partitioning temperature can be lowered, i.e. soaked between 380°C and 430°C in order to be further electro-galvanized after cooling to the room temperature. The reheating rate can be high when the reheating is made by induction heater, but that reheating rate had no effect on the final properties of the sheet.

- Then the sheet is maintained at the partitioning temperature PT for a time Pt between 10 sec and 300 sec and preferably between 10 sec and 90 sec if the sheet is hot dip coated.

In case of hot dip coated steel, the partitioning temperature PT is preferably between 430°C and 480°C. Maintaining the sheet at the partitioning temperature involves that during partitioning the temperature of the sheet remains between PT - 20°C and PT + 20°C.

Optionally, the temperature of the sheet is adjusted by cooling or heating in order to be equal to the temperature at which the sheet has to be hot dip coated.

- The optional hot dip coating can be, for example, galvanizing but all metallic hot dip coating is possible provided that the temperatures at which the sheet is brought to during coating remain less than 480°C. When the sheet is galvanized, it is done with the usual conditions. The steel sheet according to the invention can be galvannealed i.e a thermal treatment between 515°C and 580°C to alloy the Zn coating by inter-diffusion with Fe is performed after the steel is dipped in the Zn bath. The steel according to the invention can also galvanized with Zn alloys like zinc-magnesium or zinc-magnesium-aluminum.

Generally, after hot dip coating or not, the sheet is processed according to the known art. In particular, the sheet is cool to the room temperature. After partitioning and cooling to the room temperature, the steel according to the invention shall contain: at least 10% of residual austenite, 5 to 25% of intercritical ferrite and at least 65% of the sum (i.e combination) of martensite and bainite. Instead of using hot dip coating, the sheet can be coated by electrochemical methods, for example electro-galvanizing, or through any vacuum coating process, like PVD or Jet Vapor Deposition. There again, any kind of coatings can be used and in particular, zinc or zinc alloys, like zinc-nickel, zinc-magnesium or zinc-magnesium-aluminum alloys. Steel sheets according to the invention have a yield strength YS of at least 550 MPa, a tensile strength of at least 980 MPa, a uniform elongation of at least 12% and a total elongation of at least 18% can be obtained.

The following examples are for the purposes of illustration and are not meant to be construed to limit the scope of the disclosure herein:

As an example, a sheet of 1.2 mm in thickness has the following composition: C = 0.19%, Si = 1.5% Mn = 2.2%, Cr = 0.2%, Al=0.030% the balance being Fe and impurities. All the impurity elements such as Cu, Ni, B, Nb, Ti, V; etc... have a content below 0.05%. The steel was manufactured by hot and cold rolling. The theoretical Ms Transformation point of this steel is 369°C and the calculated Ac3 point is 849°C.

Samples of the sheet were heat treated by annealing, quenching and partitioning then hot dip galvanized or electro-galvanized, the microstructure were quantified and the mechanical properties were measured.

The conditions of annealing treatment are reported at table I, the microstructures obtained are summarized in table II and the mechanical properties are in table III. Examples 1 to 15 have been hot dip coated by galvanizing at 460°C (Gl) and examples 16 to 30 have been electro-galvanized (EZ) after the annealing.

The numbers bold and underlined are not according to the invention. Table I:

Samples: TA1 TA QT PT Pt

EZ and Gl °C °C °C °c sec

1 773 800 201 400 275

2 794 800 247 400 275

3 816 800 297 400 275

4 773 825 200 400 275

5 793 825 245 400 275

6 817 825 299 400 275

7 773 835 200 400 275

8 795 835 253 400 275

9 818 835 306 400 275

10 771 850 196 400 275

11 788 850 234 400 275

12 792 850 242 400 275

13 794 870 247 400 275

14 808 870 278 400 275

15 815 870 293 400 275

16 773 800 200 460 50

17 795 800 250 460 30

18 795 800 250 460 50

19 818 800 300 460 50

20 773 825 200 460 50

21 795 825 250 460 30

22 795 825 250 460 50

23 818 825 300 460 50

24 792 850 242 460 50

25 772 850 198 460 50

26 778 870 211 460 50

27 790 870 238 460 50

28 800 870 260 460 50

29 814 850 291 460 50

30 815 870 294 460 50 Table II

Table III

YS TS UE TE

Sample Steel

MPa MPa % %

1 708 1074 13 20 Invention

2 596 1059 14 21 Invention

3 518 1040 13 20 reference

4 786 1125 12 19 Invention

5 747 1078 13 20 Invention

6 637 1081 12 19 Invention

7 906 1145 8.60 16 reference

8 876 1148 9.10 16 reference

9 852 1131 9.40 17 reference

10 1145 1321 3.80 11 reference

11 1171 1316 5.70 12 reference

12 1101 1260 4.80 12 reference

13 1156 1306 6.40 12 reference

14 1057 1250 8.00 14 reference

15 1045 1210 6.00 13 reference

16 555 1074 13 20 Invention

17 559 1095 13 20 Invention

18 552 1079 13 19 Invention

19 523 1084 13 19 reference

20 625 1112 12 19 Invention

21 611 1133 13 20 Invention

22 577 1095 13 20 Invention

23 553 1137 12 18 Invention

24 1038 1199 8.70 16 reference

25 1101 1226 7.70 15 reference

26 1018 1166 8.20 15 reference

27 1067 1209 8.60 11 reference

28 1001 1181 7.60 15 reference

29 898 1184 10.00 17 reference

30 881 1179 9.90 17 reference In these tables, TA is the annealing temperature, TA1 is the lower annealing temperature limit, QT is the quenching temperature, PT the partitioning temperature, Pt the time of maintaining at the partitioning temperature, YS is the yield strength, TS is the tensile strength, UE is the uniform elongation, TE is the total elongation, F is the fraction of intercritical ferrite, A is the fraction of retained austenite, "M at QT" is the fraction of martensite when the quench temperature QT is reached and "M and B" is the sum of martensite and bainite in the final microstructure when the sheet is cooled down to the room temperature. Samples 1 , 2, 4, 5, 6, 16, 17, 18, 20, 21, 22 and 23 which are either galvanized or electro-galvanized show that in order to obtain the desired properties and more specifically the ductility properties, the annealing temperature TA has to be set accordingly with the quench temperature QT. Whatever the partitioning temperature PT chosen, the lower the TA temperature, the lower the QT temperature. Matching the TA temperature and the QT temperature allows obtaining an adequate fraction of martensite after the quench in regards to the fraction of intercritical ferrite obtain at the end of the intercritical annealing, i.e. the higher the ferrite fraction, the higher the martensite fraction for the sheet to have high strength and sufficient ductility. Samples 7 to 15 and 24 to 30 show that annealing temperatures above 830°C lead to a fraction of intercritical ferrite too small to ensure enough ductility. On the other hand, samples 3 and 19 show that if the annealing temperature is lower than the one calculated with the relation TA1 = Ac3 - 0.45 ^* (Ms - QT), the YS is lower than 550 MPa. Indeed, the fraction of martensite when the quench temperature QT is reached and consequently the fraction of combined martensite and bainite in the microstructure of the sheet when it is cooled down to the room temperature is not enough.

Samples 16, 17, 18, 20, 21 , 22 and 23 show that with a partitioning temperature of 460°C and a partition time between 10 sec and 60 sec it is possible to obtain the desired properties of the galvanized sheets.

On the other hand, samples 1 to 6 show that with a partition temperature of 400°C and a partitioning time between 10s and 300 s it is also possible to obtain the desired properties. Steel according to the invention can be used to make parts for automotive body in white.