Cold rolled steel sheet and vehicle

Abstract

A cold rolled and hot dip coated steel sheet presenting a tensile strength above 100050Al MPa, a uniform elongation above 15% and a low density is provided. The steel includes, by weight percent: 0.1C0.5%, 3.5Mn10.0%, 0Al9.0%, Si5.0%, Ti0.2%, V0.2%, Nb0.2%, S0.004%, P0.025%, 0.5Si+Al9.0%, B0.0035, Cr1%, The balance being Fe and impurities and the microstructure containing 25% to 90% of ferrite, 10% to 50% of austenite, kappa precipitates lower than 5% and martensite lower than 25%. The steel is able to be coated using total oxidation.

Claims

1. A cold rolled steel sheet comprising, by weight percent: 0.1C0.5%; 3.5Mn10.0%; Al9.0%; Si5.0%; 0.5Si+Al9.0%; Ti0.2%; V0.2%; Nb0.2%; B0.0035%; Cr1%; S0.004%; P0.025%; the remainder of the composition being iron and unavoidable impurities resulting from the smelting; the microstructure including, as a surface fraction, 10% to 50% of austenite, 25% to 90% of ferrite, less than 5% of Kappa precipitates and less than 25% of martensite; the sheet presenting from a top surface the successive following layers: a top layer of metallic iron which thickness ranges from 50 to 300 nm; and a first under-layer made of metallic iron which contains one or more precipitates of oxides chosen among Mn, Si, Al, Cr and B, which thickness ranges from 1 to 8 m.

2. The cold rolled steel sheet according to claim 1, further comprising a second under-layer, lying under said first under-layer, made of pure ferrite, which thickness ranges from 10 to 50 m.

3. The cold rolled steel sheet according to claim 1, the steel composition having a manganese content of 5.0 to 9.0%.

4. The cold rolled steel sheet according to claim 1, the steel composition having a carbon content of 0.1 to 0.3%.

5. The cold rolled steel sheet according to claim 1, the steel composition having a carbon content of 0.15 to 0.25%.

6. The cold rolled steel sheet according to claim 1, the steel composition having an aluminum content of 1.5 to 9%.

7. The cold rolled steel sheet according to claim 1, the steel composition having an aluminum content of 5 to 8%.

8. The cold rolled steel sheet according to claim 1, the steel composition having a silicon content less than or equal to 1.5%.

9. The cold rolled steel sheet according to claim 1, the steel composition having a silicon content less than or equal to 0.3%.

10. The cold rolled steel sheet according to claim 1, the steel microstructure containing from 25 to 40% of austenite.

11. The cold rolled steel sheet according to claim 1, the steel microstructure containing from 50 to 85% of ferrite.

12. The cold rolled steel sheet according to claim 1, the steel microstructure including less than 15% of martensite.

13. The cold rolled steel sheet according to claim 1, the steel microstructure being free of kappa precipitates.

14. The cold rolled steel sheet according to claim 1, wherein the steel sheet has a tensile strength TS greater than or equal to 100050% Al in MPa, a uniform elongation UE1 greater than or equal to 15% and a hole expansion HE greater than or equal to 20%.

15. A vehicle comprising: a structural part made out of a steel sheet according to claim 1.

16. A metallic coated steel sheet comprising: a cold rolled steel sheet according to claim 1; and a coating on the cold rolled steel sheet; the coating applied via hot dip coating, electro-deposition or vacuum coating.

17. The metallic coated steel sheet according to claim 16, wherein the metallic coated steel sheet is heated treated.

18. The metallic coated steel sheet according to claim 16, wherein the sheet is galvannealed.

19. The metallic coated steel sheet according to claim 16, wherein the coating is applied via electro-deposition.

20. The metallic coated steel sheet according to claim 16, wherein the coated steel sheet has a tensile strength TS greater than or equal to 100050% Al in MPa.

21. The metallic coated steel sheet according to claim 16, wherein the coated steel sheet has an elongation UE1 greater than or equal to 15%.

22. The metallic coated steel sheet according to claim 16, wherein the coated steel sheet has a hole expansion HE greater than or equal to 20%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying figures are examples and shall not be taken as limiting the scope of the present invention.

(2) The figures are such that:

(3) FIG. 1 illustrates the microstructure of example A2 after cold-rolling and annealing. The dark phase is the austenite, white phase is the ferrite,

(4) FIG. 2 illustrates the tensile curve of example A2 after cold-rolling and annealing,

(5) FIG. 3 shows GDOS profile of the example A6 that has been produced out of the invention,

(6) FIG. 4 shows GDOS profile of the example A3 that has been produced according to the invention,

(7) FIG. 5 shows the result of the 3-point bending test on the A6 example,

(8) FIG. 6 shows the result of the 3-point bending test on the A3 example,

(9) FIG. 7 shows the result of the 3-point bending test on the A4 example,

(10) FIG. 8 shows the thermal path of the annealing cycle according to the example A2, and

(11) FIG. 9 shows the Al impact on the stability of tensile strength for steel D (0.2 C 5 Mn).

DETAILED DESCRIPTION

(12) According to the invention, the chemical composition of the steel is balanced to reach the properties targets. Following chemical composition elements are given in weight percent.

(13) Aluminum content must be below 9.0%, as it must be kept strictly less than this value to avoid a brittle intermetallic precipitation.

(14) Aluminum additions are interesting for many aspects so as to increase the stability of retained austenite through an increase of carbon in the retained austenite. Moreover, the inventors have shown that, surprisingly, even though Al is supposed to stabilize ferrite, in the present invention, the higher the Al content, the better the stability of the austenite formed during annealing.

(15) The improved robustness during annealing addition of Al leads to lower variation of austenite fraction as a function of temperature during annealing on continuous annealing lines.

(16) Al is the most efficient element, able to open a large feasibility window for continuous annealing since it favours the combination of full recrystallization at annealing temperatures T.sub.anneal above the non-recrystallization temperature as well as austenite stabilization.

(17) Al also allows reducing the steel density up to 10%. Moreover, such element reduces detrimental effects of high strength steels, such as spring-back, hydrogen embrittlement and rigidity loss. As shown in FIG. 9, above 1.5% of Al, the steel robustness is improved and delta tensile strength is equal or below 10 MPa/ C. of annealing temperature. It has however an impact of the tensile strength that can be reached. It decreases the tensile strength by 50 MPa by percent of added aluminium.

(18) Just as aluminum, silicon is an element for reducing the density of steel. Silicon is also very efficient to increase the strength through solid solution. However its content is limited to 5.0%, because beyond this value, brittleness issues are met during cold-rolling.

(19) According to the invention, the carbon content is between 0.10 and 0.50%. Carbon is a gamma-former element. It promotes, with the Mn, the onset of austenite. Below 0.10%, the mechanical strength above 100050Al in MPa is difficult to achieve. If the carbon content is greater than 0.50%, the cold-rollability is reduced and the weldability becomes poor.

(20) Manganese must be between 3.5% and 10.0%. This element, also austenite-stabilizer, is used to stabilize enough austenite in the microstructure. It also has a solid solution hardening and a refining effect on the microstructure. For Mn content less than 3.5%, the stabilization of the retained austenite in the microstructure is not sufficient to enable the combination of the uniform elongation above 15% and the tensile strength above 100050% Al in MPa. Above 10.0%, weldability becomes poor. Segregations and inclusions deteriorate the damage properties.

(21) Micro-alloy elements such as titanium, vanadium and niobium may be added respectively in an amount less than 0.2%, in order to obtain an additional precipitation hardening. In particular titanium and niobium are used to control the grain size during the solidification. One limitation, however, is necessary because beyond, a saturation effect is obtained.

(22) Chromium is tolerated up to 1%. Above that limit, detrimental surface oxides may appear.

(23) Above a sulphur content of 0.004%, the ductility is reduced due to the presence of excess sulfides such as MnS which reduce the ductility, in particular during hole-expansion tests.

(24) Phosphorus is an element which hardens in solid solution but which reduces the spot weldability and the hot ductility, particularly due to its tendency to segregation at the grain boundaries or co-segregation with manganese. For these reasons, its content must be limited to 0.025%, and preferably 0.015%, in order to obtain good spot weldability.

(25) The maximum boron content allowed by the invention is 0.0035%. Above such limit, a saturation level is expected as regard to grain refinement.

(26) The balance is made of iron and inevitable impurities.

(27) To reach the targeted properties, the microstructure of the steel sheet of the invention must contain, as surface fraction, 10% to 50% of austenite, 25% to 90% of ferrite, kappa precipitates below 5% and martensite lower than 25%.

(28) Austenite is a structure that brings ductility, its content must be above 10% so that the steel of the invention is enough ductile with uniform elongation above 15% and its content must be below 50% because above that value the mechanical properties balance deteriorates.

(29) Ferrite in the invention is defined by a cubic center structure obtained from recovery and recrystallization upon annealing whether from preceding ferrite formed during solidification or from bainite or martensite of the hot rolled steel. Its content must be between 25 and 90% so as to have (100050% Al) in MPa minimum of tensile strength and at least 15% of uniform elongation.

(30) Kappa in the invention is defined by precipitates whose stoechiometry is (Fe,Mn).sub.3AlC.sub.x, where x is strictly lower than 1. The surface density of precipitates Kappa can go up to 5%. Above 5%, the ductility decreases and uniform elongation above 15% is not achieved. In addition, uncontrolled precipitation Kappa around the ferrite grain boundaries may occur, increasing, as a consequence, the efforts during hot and/or cold rolling. Preferentially, the surface density of Kappa precipitates should be less than 2%. As the microstructure is uniform, the surface fraction is equal to the volume fraction.

(31) Martensite is a structure formed during cooling after the soaking from the unstable austenite. Its content must be limited to 25% so that the hole expansion remains above 20%. In a preferred embodiment, such martensite is tempered, either after or before the coating step, depending on the type of coating.

(32) Another main characteristic of the steel sheet according to the invention lies in its reactive surface, which can be described as comprising the successive following layers:

(33) a top layer of pure metallic iron which thickness ranges from 50 to 300 nm and

(34) a first under-layer made of metallic iron which contains also one or more precipitates of oxides chosen among Mn, Si, Al, Cr and B, which thickness ranges from 1 to 8 m.

(35) Such a structure guarantees reactivity during the phosphate conversion treatment of the bare steel, a good wetting and adherence of metallic coatings such as zinc or aluminium coatings. This improves the ability for electro-deposition of paint.

(36) As long as such surface is obtained, any suitable manufacturing method can be employed.

(37) By example, one method to produce the steel according to the invention implies casting steel with the chemical composition of the invention.

(38) The cast steel is reheated between 1100 C. and 1300 C. When slab reheating temperature is below 1100 C., for Al<4 wt %, the rolling loads increase too much and hot rolling process becomes difficult; for Al4 wt %, the last hot rolling pass is hardly kept above 800 C. due to thermal losses during the rolling process. Above 1300 C., oxidation is very intense, which leads to scale loss and surface degradation.

(39) The reheated slab can then be hot rolled with a temperature between 1250 C. and 800 C., the last hot rolling pass taking place at a temperature T.sub.lp above or equal to 800 C. If T.sub.lp is below 800 C., hot workability is reduced.

(40) The steel is cooled at a cooling speed V.sub.cooling1 of at least 10 C./s until the coiling temperature T.sub.coiling lower or equal to 700 C. If the cooling speed V.sub.cooling1 is below 10 C./s, in the case where Al4 wt %, and Mn4 wt %, there is a precipitation of harmful Kappa precipitates at the interfaces between ferrite and austenite.

(41) T.sub.coiling must be lower or equal to 700 C., If the coiling temperature is above 700 C., there is a risk to form a coarse microstructure consisting of: coarse ferrite and bainite structure when Al content is below 4 wt %; and Kappa carbides at the interfaces between ferrite and austenite when Al content is above or equal to 4 wt % Al and Mn above 4% Mn.

(42) The steel is then cold rolled with a cold rolling ratio between 30% and 75% so as to obtain a cold rolled steel. Below 30%, the recrystallization during subsequent annealing is not favoured enough and the uniform elongation above 15% is not achieved due to a lack of recrystallization. Above 75%, there is a risk of edge cracking during cold-rolling.

(43) Then, the steel is heated at a heating rate H.sub.rate at least equal to 1 C./s up to the annealing temperature T.sub.anneal. If the heating rate is below 1 C./s, the force for recrystallization is too low, hindering the achievement of the target microstructure.

(44) During the heating, from 550 C. up to the end of soaking at T.sub.anneal, the steel goes through an oxidizing atmosphere so as to produce predominantly an iron oxide with a thickness between 100 and 600 nm. If the iron oxide is thinner than 100 nm, the iron oxide will disappear too early, allowing again external selective oxidation of the alloying elements during the subsequent reductive annealing, hindering reactivity of the surface during the coating process, If the iron oxide is above 600 nm, the risk of non-adherent iron oxides is given polluting the hearth roll of the furnace by pick-up issues and leading thus by indentation to surface defects. A thickness greater than 600 nm can also lead to an only partial reduction of the iron oxide during the soaking or cooling, or soaking and cooling step when a reductive atmosphere is applied.

(45) If radiant tubes are used in the furnace for heating, the atmosphere for iron reduction shall contain between 2 and 8% H.sub.z, the balance being nitrogen and unavoidable impurities: If the H.sub.2 content is lower than 2%, reduction ability of the atmosphere is too low to reduce completely the iron oxide. If the H.sub.2 content is higher than 8%, the reduction process is complete, but no more economically viable.

(46) The steel is then annealed at a temperature T.sub.anneal between T.sub.min C. and T.sub.max C. during 30 and 700 seconds. Controlling the annealing temperature is an important feature of the process since it enables to control the austenite fraction and its chemical composition. The annealing temperature should be high enough to form more than the 10% retained austenite required in the final microstructure and to avoid precipitation of more than 5% Kappa carbides. The annealing temperature should not be too high to avoid the formation of more than 50% austenite and to avoid grain coarsening leading to a tensile strength below 100050Al (%) when Al4 wt %. The annealing temperature should also be sufficiently high to enable the sufficient recrystallization of the cold-rolled structure. As the phase transformations depend on the chemical composition, the preferred T.sub.anneal is defined as the following preferably: The annealing temperature T.sub.min is defined such as: T.sub.min=72136*C20*Mn+37*Al+2*Si, (in C.). Below this temperature, the minimum austenite fraction is not formed, or its stability is too high, leading to a limited tensile strength. The annealing temperature T.sub.max is defined such as: T.sub.max=690+145*C6.7*Mn+46*Al+9*Si (in C.). Above T.sub.max, there is also a risk to form too many martensite, leading to a limited uniform elongation and hole expandability.

(47) During the soaking at T.sub.anneal down to 600 C., the steel goes through an atmosphere containing between 2% and 35% H.sub.2, the balance being nitrogen and unavoidable impurities, so as to reduce the iron oxide formed upon heating applying a dew point below the critical dew point for iron oxidation typically below 10 C. If the H.sub.2 content is lower than 2%, reduction ability of the atmosphere is too low to reduce completely the iron oxide. If the H.sub.2 content is higher than 35%, the reduction process is complete, but no more economically viable.

(48) Preferably, the dew point during iron reduction is below 30 C., so as to allow fast reduction kinetics.

(49) In a preferred embodiment, H.sub.2 content is higher than 20% but lower than 35%.

(50) In another embodiment, the reduction step is by-passed and the iron oxide is removed by pickling (formic acid, chlorohydric acid, sulphuric acid) after the whole annealing treatment is completed. This is because, if the steel does no go through a reductive atmosphere, slight re-oxidation may take place and this layer shall be removed. In the invention: First part of the soaking means the heating and up to 90% of the soaking time While the second part of the soaking means the remaining soaking time and the cooling from the annealing temperature down to 600 C.

(51) The steel is then cooled at a cooling rate V.sub.cooling2 of typical annealing lines, preferably, this cooling rate is above 5 C./s and below 70 C./s. If the cooling rate is below 5 C./s, there is a risk to form more than 5% of Kappa carbides when Al content is above 4 wt %. The cooling atmosphere contains between 2% and 35% H2 so as to avoid re-oxidation of the reduced iron oxide formed applying a dew point below the critical dew point for iron oxidation typically below 10 C.

(52) Optionally, the steel is cooled down at V.sub.cooling2 to a temperature T.sub.OA between 350 C. and 550 C. and kept at T.sub.OA for a time between 10 and 300 seconds. It was shown that such a thermal treatment to facilitate the Zn coating by hot dip process for instance does not affect the final mechanical properties.

(53) The steel is further cooled at a cooling rate V.sub.cooling3 of typical annealing lines down to room temperature, preferably, this cooling rate is above 5 C./s and below 70 C./s to obtain a cold rolled and annealed steel.

(54) In another embodiment, after maintaining the steel at T.sub.OA, the steel is hot dip coated with Zn or Zn alloys meaning that Zn content is the highest in the alloy in percent.

(55) In another embodiment, after maintaining the steel at T.sub.OA, the steel is hot dip coated with Al or Al alloys meaning that Al content is the highest in the alloy in percent.

(56) Optionally, the cold rolled and annealed steel is tempered at a temperature T.sub.temper between 200 and 400 C. for a time t.sub.temper between 200 and 800 seconds. This treatment enables the tempering of martensite, which might be formed during cooling after the soaking from the unstable austenite. The martensite hardness is thus decreased and the hole expandability is improved. Below 200 C., the tempering treatment is not efficient enough. Above 400 C., the strength loss becomes high and the balance between strength and hole expansion is not improved anymore.

(57) In another embodiment, the cold rolled and annealed steel undergoes a phosphate conversion treatment.

(58) In another embodiment, the cold rolled and annealed steel is coated by Zn, Zn-alloys, Al or Al alloys applied by electrodeposition or vacuum technologies. Zn alloys and Al alloys meaning that respectively, Zn and Al are major constituents of the coating.

(59) Semi-finished products have been developed from a steel casting. The chemical compositions of semi-finished products, expressed in weight percent, are shown in Table 1 below. The rest of the steel composition in Table 1 includes or consists of iron and inevitable impurities resulting from the smelting.

(60) TABLE-US-00001 TABLE 1 Chemical composition (wt %). Steel C Mn Al Si Cr Si + Al Comment A 0.21 8.2 7.4 0.26 0.02 7.66 Invention B 0.2 3.8 0 1.5 0.3 1.5 Invention C 0.15 1.9 0.05 0.2 0.2 0.25 Comparative example D 0.196 5.01 1.03 0.012 <0.010 1.042 Invention E 0.189 5.01 2.85 0.02 <0.010 2.87 Invention F 0.2 4 6.2 <0.050 <0.010 6.2 Invention G 0.19 6.2 6 <0.050 <0.010 6 Invention H 0.12 5.15 2.31 0.509 <0.010 2.819 Invention Steel S P Ti V Nb Comment A <0.005 <0.025 <0.010 <0.010 <0.010 Invention B <0.005 <0.025 <0.010 <0.010 <0.010 Invention C <0.005 <0.025 <0.01 <0.01 <0.01 Comparative example D 0.002 0.022 <0.010 <0.010 <0.010 Invention E 0.0021 0.02 <0.010 <0.010 <0.010 Invention F 0.0031 0.02 <0.010 <0.010 <0.010 Invention G 0.004 0.017 <0.010 <0.010 <0.010 Invention H <0.005 0.017 <0.010 <0.010 <0.010 Invention

(61) These steels are boron free.

(62) The products have first been hot-rolled. The hot rolled plates were then cold rolled and annealed. The production conditions are shown in Table 2 with the following abbreviations:

(63) T.sub.reheat: is the reheating temperature;

(64) T.sub.lp is the finishing rolling temperature;

(65) V.sub.cooling1: is the cooling rate after the last rolling pass;

(66) T.sub.coiling: is the coiling temperature;

(67) Rate: is the rate of cold rolling reduction;

(68) H.sub.rate: is the heating rate;

(69) T.sub.anneal: is the soaking temperature during annealing;

(70) t.sub.anneal: is the soaking duration during annealing;

(71) V.sub.cooling2: is the cooling rate after the soaking;

(72) t.sub.OA: is the time during which the plate is maintained at a temperature T.sub.OA;

(73) V.sub.cooling3: is the cooling rate below T.sub.OA.

(74) TABLE-US-00002 TABLE 2 Hot-rolling and cold-rolling and annealing conditions T.sub.reheat T.sub.lp V.sub.cooling1 T.sub.cooling Rate H.sub.rate T.sub.anneal t.sub.anneal V.sub.cooling2 T.sub.OA t.sub.OA V.sub.cooling3 ( C.) ( C.) ( C./s) ( C.) (%) ( C./s) ( C.) (s) ( C./s) ( C.) (s) ( C./s) A1 1180 905 50 500 74 15 830 136 50 50 A2 1180 964 50 500 74 15 850 136 50 50 A3 1180 964 50 500 74 15 790 136 50 50 A4 1180 964 50 500 74 15 900 136 50 50 A5 1180 964 50 500 74 15 850 136 50 50 A6 1180 964 50 500 74 15 900 136 50 50 A7 1180 964 50 500 74 15 900 136 50 50 A8 1180 964 50 500 74 15 830 136 50 50 B1 1250 900 30 550 50 5 790 130 20 470 38 20 B2 1250 900 30 550 50 5 790 130 20 470 38 20 B3 1250 900 30 550 50 5 675 130 20 470 38 20 C1 1250 900 30 550 60 10 800 60 20 460 10 20 D1 1250 930 15 600 50 16 710 120 20 400 300 5 E1 1250 930 15 600 50 16 770 120 20 400 300 5 F1 1200 950 60 450 75 15 900 136 50 410 500 20 F2 1200 950 60 450 75 15 900 136 50 410 500 20 F3 1200 950 60 450 75 15 900 136 50 410 500 20 F4 1200 950 60 450 75 15 900 136 50 410 500 20 G1 1200 950 60 450 75 15 850 136 50 410 500 20 G2 1200 950 60 450 75 15 850 136 50 410 500 20 H1 1200 900 10 600 50 10 770 120 20 410 500 5

(75) The products were annealed under different annealing atmospheres. In Table 3, the annealing atmospheres are presented, and the indication of pickling in formic acid after the complete continuous annealing cycle. Yes if a pickling treatment was applied, No if no pickling treatment was applied.

(76) If the annealing atmosphere from 550 C. up to the end of soaking at T.sub.anneal was oxidizing for iron by adjusting the dew point and the hydrogen content, the indication Oxidizing was set in the column Atmosphere from 550 C. up to the end of soaking at T.sub.anneal; If the atmosphere was reducing for iron, Reducing was set. Additionally, the H2 content and the dew point of the annealing atmosphere are given.

(77) If the annealing atmosphere during the soaking at T.sub.anneal down to 600 C. was reducing for iron oxide, the indication Reducing was set in the column Atmosphere during the soaking at T.sub.anneal down to 600 C.. If the annealing atmosphere was oxidizing for iron, oxidizing is indicated. Additionally, the H2 content and the dew point of the annealing atmosphere are given.

(78) In table 3 here below, EG stands for electro-galvanized while GI stands for galvanized.

(79) TABLE-US-00003 TABLE 3 Annealing conditions to create the proper reactive surface after annealing, balance N2 Atmosphere during the second part Pickling in formic acid Atmosphere from 550 C. up to the of soaking at Tanneal down to after the continuous coating Steel end of the first part of the soaking 600 C. annealing type A1 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG A2 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG A3 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG A4 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG A5 Oxidizing - Dew point +30 C., 5% H2 Oxidizing- Dew point +30 C., 5% H2 No EG A6 Reducing- Dew point 40 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG A7 Oxidizing - Dew point +30 C., 5% H2 Oxidizing - Dew point +30 C., 5% H2 Yes EG A8 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No GI B1 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No GI B2 Reducing- Dew point 40 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No GI B3 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No GI C1 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No GI D1 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG E1 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG F1 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG F2 Reducing- Dew point 40 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG F3 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No GI F4 Reducing- Dew point 40 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No GI G1 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG G2 Reducing- Dew point 40 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG H1 Oxidizing - Dew point +30 C., 5% H2 Reducing - Dew point 40 C., 5% H2 No EG

(80) Samples A6, B2, F2, F4 and G2 have been annealed under a regular reducing atmosphere (dew point=40 C., 5% H2) giving rise to bad surface reactivity. The GDOS profile of such surfaces is characterized by a first zone where the Fe signal is very low while the O signal is high, reaching more than 50% at the free surface. In that zone, Mn enrichment is also detected. Below that layer the Fe signal increases and the O signal decreases at a rate of about 1% per nanometer. This oxygen signal tail is typical of the presence of an external selective oxide layer, which oxygen atoms are partly sputtered and partly implanted into the substrate during the measurement. Some superficial pollution is visible due to the transfer of the samples from the annealing simulator to the GDOS analysis. At FIG. 3, In (A) some superficial pollution is visible due to the transfer of the samples from the annealing simulator to the GDOS analysis.

(81) Table 4 presents the following characteristics:

(82) Ferrite: OK refers to the presence of ferrite with a volume fraction between 25 and 90% in the microstructure of the annealed sheet. KO refers to comparative examples where ferrite fraction is outside this range.

(83) Austenite: OK refers to the presence of austenite with a volume fraction between 10 and 50% in the microstructure of the annealed sheet. KO refers to comparative examples where austenite fraction is outside this range.

(84) Martensite: OK refers to the presence or not of martensite with a volume fraction less than 25% in the microstructure of the annealed sheet. KO refers to comparative examples where martensite fraction is above 25%.

(85) K: OK refers to the presence or not of precipitates in the microstructure Kappa with a surface fraction of less than 5%. This measurement is performed with a scanning electron microscope. When it says KO, fraction of kappa precipitates is above 5%.

(86) UTS (MPa) refers to the tensile strength measured by tensile test in the longitudinal direction relative to the rolling direction.

(87) UEl (%) refers to the uniform elongation measured by tensile test in the longitudinal direction relative to the rolling direction.

(88) HE (%): refers to the hole expansion ratio according to the norm ISO 16630 2009. The method of determining the hole expansion ratio HE % is used to evaluate the ability of a metal to resist to the forming of a cut-edge. It consists in measuring the initial diameter D.sub.i of the hole before forming, then the final hole diameter D.sub.f after forming, determined at the time of through-cracks observed on the edges of the hole. It then determines the ability to hole expansion HE % using the following formula:

(89) $\mathrm{HE}\%=100\frac{\left(\mathrm{Df}-\mathrm{Di}\right)}{\mathrm{Di}}$
Under this method, the initial hole diameter is of 10 millimeters.

(90) TABLE-US-00004 TABLE 4 Properties of cold-rolled and annealed sheets Steel Ferrite Austenite martensite K TS (MPa) UEI (%) HE (%) A1 OK (81%) OK (17%) OK (0%) OK (2%) 831 15 30% A2 OK (80%) OK (20%) OK OK (0%) 800 15 42 A3 OK OK (15%) OK (0%) KO (>5%) Not Not Not measured measured measured A4 OK OK (25%) OK OK (0%) 730 20 Not measured A5 OK (80%) OK (20%) OK OK (0%) 800 15 42 A6 OK OK (25%) OK OK (0%) 730 20 Not measured A7 OK OK (25%) OK OK (0%) 730 20 Not measured A8 OK (81%) OK (17%) OK (0%) OK (2%) 831 15 30% B1 KO KO (8%) KO (92%) OK (0%) Not Not Not measured measured measured B2 KO KO (8%) KO (92%) OK (0%) Not Not Not measured measured measured B3 OK (60%) OK (30%) OK (10%) OK (0%) 1092 17 30 C1 OK (40%) KO (0%) OK (10%) OK (0%) 820 14 23 D1 OK (50%) OK (28%) OK (22%) OK (0%) 1075 22.8 Not measured E1 OK (66%) OK (32%) OK (2%) OK (0%) 1023 24.4 Not measured F1 OK (79%) OK (21%) OK (0%) OK (0%) 723 25 Not measured G1 OK (74%) OK (26%) OK (0%) OK (0%) 702 20 Not measured H1 OK (69%) OK (23%) OK (8%) OK (0%) 965 16 Not measured

(91) B1 has not been measured due to brittle behaviour. For C1, the rest of the microstructure (50%) is made of bainite. C1 presents a tensile strength of 820 MPa which is too low for the invention.

(92) Table 5 presents the results of coatability by electro deposition of a Zinc coating.

(93) The targeted surface and subsurface micro structure is indicated as OK if the surface is made of an external layer of metallic iron, thickness ranging from 50 to 300 nm, covering an internal layer made of metallic iron and containing precipitates of internal oxides of Mn, Al, Si, Cr and B and other elements more oxidizable than iron, which thickness ranges from 1 to 8 m, superimposed onto a decarburized layer, mainly made of ferrite, which thickness ranges from 10 to 50 m. If the surface and subsurface differs from the targeted surface, the microstructure is judged unsufficient KO.

(94) The coating quality is characterised by the covering ratio and the coating adherence.

(95) The covering ratio is indicated as OK, when full coverage is observed by the naked eye, and KO if coating defects such as uncoated areas or bare spots are observed.

(96) The coating adherence was tested in a 3-point bending test (180) on 1 mm sheets using a 3 mm punch with a tip of 1.5 mm in radius. The adherence is judged excellent OK if no peeling of the zinc coating is observed after applying and withdrawing of an adhesive scotch tape. If peeling or flaking of the coating is observed, the adherence is judged insufficient KO.

(97) TABLE-US-00005 TABLE 5 Surface properties of cold-rolled and annealed and coated sheets Targeted surface and subsurface Covering Coating Coating micro structure ratio adherence type A1 OK OK OK EG Invention A2 OK OK OK EG Invention A3 OK OK OK EG Invention A4 OK OK OK EG Invention A5 KO KO KO EG reference A6 KO KO KO EG reference A7 OK OK OK EG Invention A8 OK OK OK GI Invention B1 OK OK OK GI Invention B2 KO KO KO GI reference B3 OK OK OK GI Invention C1 OK OK OK GI Invention D1 OK OK OK EG Invention E1 OK OK OK EG Invention F1 OK OK OK EG Invention F2 KO KO KO EG reference F3 OK OK OK GI Invention F4 KO KO KO GI reference G1 OK OK OK EG Invention G2 KO KO KO EG reference H1 OK OK OK EG Invention

(98) In FIG. 5, the coating adherence was tested in a 3-point bending test (180) on 1 mm sheets using a 3 mm punch with a tip of 1.5 mm in radius. Non-adherence of zinc coating is observed for steel example A6 (out of the invention). At (a), a coated part is visible, which was under low solicitation during the bending test. At (b), the steel substrate is visible after peeling off of coating; this part was under high solicitation in the bending test.

(99) Sheets A1, A2, A3, A4, A7, A8, B1, B3, C1, D1, E1, F1, F3, G1 and H1 are sheets whose chemical composition and processing method are according to the invention.

(100) For the sample A3, the production has been carried out under an oxidizing atmosphere (dew point=+30 C.) followed by a reducing atmosphere. The surface is made of a first layer where the Fe GDOS signal reaches a maximum and the oxygen one a minimum as shown in FIG. 4. This layer (B) is made of metallic iron. The second layer (C) is characterized by a continuous decrease of the oxygen signal at a slow rate, around 1% per 100 nm and corresponds to a zone where internal selective oxides of Mn and Al have precipitated. It extends up to an oxygen level of 5% which corresponds here to a thickness of 4 m. In (A) some superficial pollution is visible due to the transfer of the samples from the annealing simulator to the GDOS analysis.

(101) For sample A3, the coating adherence was tested in a 3-point bending test (180) on 1 mm sheets using a 3 mm punch with a tip of 1.5 mm in radius. Very good adherence of the zinc coating is observed for steel example A3 (within the invention) as shown in FIG. 6. At (c), a coated part is visible, which was under low solicitation during the bending test. At (d), the coating is showing excellent adherence, this part was under high solicitation in the bending test.

(102) The coating adherence was also tested in a 3-point bending test (180) on 1 mm sheets using a 3 mm punch with a tip of 1.5 mm in radius for A4 as shown in FIG. 7. Very good adherence of the zinc coating is observed for steel example A4 (within the invention). At (e), a coated part is visible, which was under low solicitation during the bending test. At (f), the coating is showing excellent adherence, this part was under high solicitation in the bending test.

(103) The microstructure of the sheet A1 is illustrated by FIG. 1. Its tensile curve is shown on FIG. 2.

(104) B2 is not according to the invention, due to untargeted microstructure and coating method. Its annealing temperature is out of target.

(105) A5 did not undergo a pickling step while it has undergone only oxidation during annealing; as a consequence coating adherence and covering ratio are bad.

(106) A6, B2, F2, F4 and G2 have undergone only reduction during the annealing; as a consequence, coating adherence and covering ratio results are bad.

(107) For the steels according to the invention, in addition to good coatability via electro-galvanization (EG) or galvanization, the tensile strengths are higher than 100050Al MPa, and their uniform elongation is greater than 15%. Furthermore, hole expansion is above 20% also.

(108) The steel sheets according to the invention will be beneficially used for the manufacture of structural or safety parts in the automobile industry.