High strength zinc coated steel sheet, and method for manufacturing the same.

Abstract

The invention relates to a high strength zinc coated steel sheet made of a steel having a composition comprising, by weight percent C: 0.20 - 0.35 %, Mn: 0.5 – 3.0 %, B: 0.0005 – 0.005%, Si 0.01- 1.20 %, Cr 0.01 - 1.5 % S ≤ 0.010 %, P ≤ 0.020 %, N ≤ 0.008 % and comprising optionally one or more of the following elements, in weight percentage: Al ≤ 1%, Mo ≤ 0.4 %, Ti ≤ 0.05 %, Nb ≤ 0.05 %, said steel sheet having a microstructure comprising, in area fraction, bainite and tempered martensite, the sum being comprised from 25% to 65%, the rest being fresh martensite and martensite-austenite islands.

Description

[0001] High strength zinc coated steel sheet, and method for manufacturing the same.

[0002]

[0001] The present invention relates to a high strength and weldable zinc coated steel sheet, and to a method to obtain such steel sheet.

[0003]

[0002] To manufacture various items such as parts of body structural members and body panels for automotive vehicles, it is known to use sheets made of DP (Dual Phase) steels or TRIP (Transformation Induced Plasticity) steels.

[0004]

[0003] One of the major challenges in the automotive industry is to decrease the weight of vehicles in order to improve their fuel efficiency in view of the global environmental conservation, without neglecting the safety requirements. To meet these requirements, new high strength steels are continuously developed by the steelmaking industry, to have sheets with improved yield and tensile strengths, and good ductility and formability.

[0005]

[0004] These steels are usually coated with a metallic coating improving properties such as corrosion resistance. The metallic coatings can be deposited by hot dip galvanizing after the annealing of the steel sheets. Nevertheless, it is difficult to achieve an ultra-high strength galvanized steel sheet, with good weldability, and a limiting risk of hydrogen embrittlement, due to hydrogen absorption during the heating before entering in the zinc bath.

[0006]

[0005] The aim of the present invention is therefore to remedy this drawback by providing a steel sheet having a combination of high mechanical properties with a tensile strength TS above or equal to 1450MPa, having a good resistance to hydrogen absorption and a good weldability. Preferably the hydrogen content of the zinc coated steel sheet is below or equal to 0.3ppm.

[0007]

[0006] The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet can also comprise the characteristic of any one of claims 2 and 3. Another object is achieved by providing the method according to claim 4. Another object is achieved by providing the method according to claim 5. The method can also comprise any of the characteristics of claim 6. Another object is achieved by providing a resistant spot weld according to any one of claims 7 and 8.

[0008]

[0007] Other characteristics and advantages of the invention will be described in greater detail in the following description.

[0009]

[0008] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive.

[0009] The composition of the steel sheet according to the invention will now be described, the content being expressed in weight percent (wt. %).

[0010]

[0010] According to the invention, the carbon content is from 0.20% to 0.35 % to ensure a satisfactory strength and good weldability properties. Above 0.35% of carbon, weldability of the steel sheet may be reduced. If the carbon content is lower than 0.20%, the strength of the tempered martensite is not sufficient to get TS above 1450MPa. Preferably, the carbon content is from 0.20% to 0.30%, more preferably from 0.22% to 0.30%.

[0011]

[0011] The manganese content is from 0.5% to 3.0 %. Above 3.0% of addition, weldability of the steel sheet may be reduced. Moreover, the risk of central segregation increases to the detriment of the mechanical properties. Preferably, the manganese content is from 0.5% to 2.5%, more preferably from 0.5% to 2.0%, even more preferably from 1.0 to 2.0%.

[0012]

[0012] Silicon content is added in a content of 0.01% to 1.20% to improve the stabilization of austenite and increase the strength. Above 1.20%, silicon is very detrimental to liquid metal embrittlement resistance. In addition, silicon oxides form at the surface, which impairs the coatability of the steel. Preferably, a minimum of 0.05% of silicon content is added. More preferably a minimum of 0.10% of silicon is added, or even 0.20% or 0.30%. The maximum amount of silicon added is preferably 1.10%, more preferably 1.0%.

[0013]

[0013] According to the invention, the chromium content is from 0.01 % to 1.5% to improve the stabilization of austenite thus retard bainite transformation and resist martensite tempering during overaging. Preferably the minimum amount of chromium is 0.10%. More preferably the minimum amount of chromium is 0.20%, even more preferably 0.30% or 0.40%.

[0014]

[0014] According to the invention, the boron content is from 0.0005% to 0.005% to retard ferrite and bainite phase transformations of the cold rolled steel sheet and to improve the spot weldability of the zinc coated sheet. Above 0.005%, the formation of boro-carbides at the prior austenite grain boundaries is promoted, making the steel more brittle. Preferably, the boron content is comprised from 0.001% to 0.005%, more preferably from 0.001% to 0.004%.

[0015]

[0015] Optionally some elements can be added to the composition of the steel according to the invention.

[0016]

[0016] Aluminium content can be added up to 1.0% to decrease the manganese segregation during casting and to improve weldability. Aluminium is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Above 1.0% of addition, the weldability of the steel sheet may be reduced, so as castability. Preferably, the minimum amount of aluminium added is 0.010%. Preferably, the maximum amount of aluminium is 0.8%, or more preferably 0.6%, even more preferably 0.5%.

[0017]

[0017] Molybdenum content can be added up to 0.4% to decrease the manganese segregation during casting and to stabilize the retained austenite thus reducing austenite decomposition during tempering. Above 0.4%, the addition of molybdenum is costly and ineffective in view of the properties which are required. Preferably, the maximum amount of molybdenum added is 0.3%.

[0018]

[0018] Titanium can be added up to 0.050 % to provide precipitation strengthening. Preferably, a minimum of 0.010% of titanium is added in addition of boron to protect boron against the formation of BN.

[0019]

[0019] Niobium can be added up to 0.05% in order to provide precipitation strengthening, refine prior austenite grain size and to resist martensite tempering.

[0020]

[0020] The remainder of the composition of the steel is iron and unavoidable impurities resulting from the smelting process and depending on the process route. In the case of a production route using a blast furnace, the level of unavoidable impurities is very low. In the case of a production route using an Electric Arc Furnace loaded with scraps, the steel sheet can further comprise residual elements coming from such scraps such as Antimony, Arsenic, Copper, Nickel, Tin, and Lead, up to 0.03% which are considered as unavoidable impurities.

[0021]

[0021] P, S and N are also part of the unavoidable impurities whatever the process route. Their content is less than or equal to 0.020 % for P, less than or equal to 0.010 % for S, and less than or equal to 0.012 % for N.

[0022]

[0022] The microstructure of the zinc coated steel sheet according to the invention will now be described. It contains, in area fraction:

[0023] - bainite and tempered martensite, the sum of the two being comprised from 25% to 65%,

[0024] - the rest being fresh martensite and martensite-austenite islands (hereinafter M-A islands).

[0025]

[0023] Bainite and tempered martensite are formed before the steel sheet enters in the zinc bath and are preserved during the galvanizing step and the subsequent cooling. This is an important condition of the invention to obtain the targeted tensile strength after the zinc bath, and to limit the hydrogen absorption during the galvanizing step. Indeed, the bainite grains in contact with each other enable hydrogen to be degassed, this phenomenon is known as the percolation. Below 25%, the hydrogen in the steel sheet cannot be degassed and the risk of fracture is increased. Above 65% of bainite and tempered martensite, the tensile strength of the steel is reduced.

[0026]

[0024] Bainite is formed during the overaging step, which is the maintaining step to a temperature Tc or T3 comprised from (Ms-50°C) to 450°C for the holding time tcfrom 3s to 1000s, Ms being the martensite start temperature. Preferably the bainite amount is from 25% to 65%.

[0027]

[0025] Martensite is formed when the cooling to Tc is below Ms and is then tempered during the overaging step. Preferably the amount of tempered martensite is below 50%, more preferably below 25%.

[0028]

[0026] The rest of the microstructure is fresh martensite and M-A islands. Both are formed from austenite formed during the soaking of the steel sheet at a temperature T2 above or equal to Ae3, Ae3 designating the equilibrium transformation temperature above which the microstructure is fully transformed into austenite. During the subsequent cooling to Tc, a part of austenite is turned into the martensite mentioned above, and a part of austenite remains. Such austenite is then transformed into fresh martensite and M-A islands during the cooling of the steel sheet after exiting the zinc bath.

[0029]

[0027] The zinc coated steel sheet according to the invention can be produced by any appropriate manufacturing method and the man skilled in the art can define one. It is however preferred to use the method according to the invention comprising the following steps:

[0030]

[0028] A semi-product able to be further hot rolled, is provided with the steel composition described above. The semi product is heated to a temperature Treheat from 1150°C to 1300°C, so to make it possible to ease hot rolling, with a final hot rolling temperature FRT from 800°C to 1000°C. Preferably, the FRT is from 800°C to 950°C, more preferably from 830°C to 950°C, even more preferably from 850°C to 950°C.

[0031]

[0029] The hot-rolled steel sheet is then cooled and coiled at a temperature TCOii below or equal to 700°C, and preferably from 300°C to 600°C.

[0032]

[0030] The hot rolled steel sheet is then cooled to a room temperature. The steel sheet can then be pickled.

[0033]

[0031] The hot rolled steel sheet can be heated up to a temperature T1 comprised from 500°C to 650°C, and maintained at said temperature for a holding time ti of 0.1 h to 120h in order to facilitate the subsequent cold rolling step, before being cooled to room temperature. Such annealing can be performed by batch annealing.

[0032] The steel sheet is then cold rolled. The reduction ratio is comprised from 5% to 80%, depending on the targeted thickness.

[0034]

[0033] The cold rolled steel sheet is heated to a temperature T2 above or equal to Ae3, and maintained at said temperature for a holding time t2 of 1 to 3600s, in order to obtain a fully austenite structure. Preferably, t2 is from 1 to 1800s, more preferably from 1 to 900s.

[0035]

[0034] The steel sheet is then cooled directly from T2 to a temperature Tccomprised from (Ms-50°C) to 450°C, preferably at a cooling rate vccomprised from 10°C / s to 100°C / s to avoid ferrite formation. In one embodiment of the invention, the steel sheet is then maintained at said Tctemperature for a holding time tcof 3s to 1000s. This maintaining step means that the temperature is kept in a range comprised from (Tc- 20°C) to (Tc+20°C) during this holding time tc. In a second embodiment of the invention, after the cooling to Tc, the steel sheet is reheated to a temperature T3 higher than Tc, and below or equal to 450°C, and maintained at said T3 temperature for the holding time tcof 3s to 1000s. This maintaining step means that the temperature is kept in a range from (T3-20°C) to (T3 +20°C) during this holding time tc. Hereinafter, the step of maintaining during tcis called an overaging step. This overaging aims to temper the martensite and to form bainite, in order to obtain the targeted tensile strength after the zinc bath, and to limit the hydrogen absorption. Preferably, tcis comprised from 10s to 1000s, more preferably from 50s to 1000s, even more preferably from 60s to 1000s, or from 60s to 500s.

[0036]

[0035] The steel sheet is then hot dip coated with a zinc coating, in a bath having a temperature comprised from 400°C to 500°C, before being cooled to room temperature.

[0036] Optionally, the zinc coated steel sheet can be heated to a temperature T4 comprised from 100°C to 300°C and maintained at said temperature for a holding time t4 of 0.05min to 2400min. This last heating step, called a tempering step, can be for example, a paint baking step. Preferably T4 is below Ms.

[0037]

[0037] The zinc-coated steel sheet according to the invention has a tensile strength TS above or equal to 1450MPa, a good resistance to hydrogen absorption and a good weldability. Preferably, the hydrogen content of the zinc-coated steel sheet is below or equal to 0.3 ppm.

[0038]

[0038] Four grades, whose compositions are gathered in table 1 , were cast in semiproducts and processed into steel sheets. Table 1 - Compositions

[0039]

[0039] The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent (wt.%).

[0040] 5

[0041] Underlined values: out of the invention

[0042]

[0040] Ae3 of the steel sheet has been determined through thermodynamic calculations with software as Thermo-calc®. Ms temperature has been determined by 10 dilatometry tests.

[0043] Table 2 - Process parameters of the zinc-coated and optionally tempered steel sheets

[0044] 15

[0041] Steel semi-products, as cast, were reheated at 1200°C, hot rolled with a finish rolling temperature FRT, coiled at a Tcoii temperature and cooled to room temperature. The steel sheets are then heated to Ti and maintained at said temperature for a holding time ti, before being cooled and cold rolled. The steel sheets are then heated to a temperature T2 temperature and maintained at said temperature for a holding time t2,

[0045] 20 before being cooled to Tc, with a cooling rate vc. The steel sheets are maintained at said temperature for a holding time tc. In trials 8 and 9, the steel sheets are submitted to a reheating to T3, before the holding step. Steel sheets are then subjected to a galvanizing step in a zinc bath at a temperature Tzn during tznand are cooled to room temperature. Some steel sheets are then tempered at a temperature T4and maintained

[0046] 25 at said T4 temperature for a holding time t4. The following specific conditions to obtain the annealed and tempered steel sheets were applied:

[0047] Underlined values: out of the invention

[0048] The steel sheets were analyzed, and the corresponding microstructure are gathered in table 3.

[0049] 5

[0050] Table 3 - Microstructure of the zinc-coated steel sheets

[0051]

[0042] The phase percentages of the microstructure of the steel sheets were determined after the cooling to room temperature at the exit of the zinc bath, before the

[0052] 10 optional tempering.

[0053]

[0043] The area fractions of phases in the microstructure are determined through the following method: a specimen is cut from the steel sheet, polished and etched with a reagent known per se, to reveal the microstructure. The section is afterwards examined through scanning electron microscope, for example with a Scanning Electron

[0054] 15 Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification above or equal to 5000x, in secondary electron mode.

[0055]

[0044] The determination of the area fraction of bainite, tempered martensite, fresh martensite and M-A islands is performed thanks to SEM observations after Nital or Picral / Nital reagent etching.

[0056] 20

[0057]

[0045] The hydrogen content in the zinc-coated steel sheets, before the optional tempering step, is calculated through a model as disclosed in the publication WO / 2023 / 111771 and is gathered in the table 3.

[0058] Table 4 - Mechanical properties of the zinc-coated and optionally tempered steel sheet

[0059]

[0046] The tensile strength TS, expressed in MPa and measured according to ISO standard ISO 6892-1, published in October 2009, are gathered in the following table:

[0047] Trials 1 -7 are according to the invention. Thanks to the microstructure of 25- 65% of bainite and tempered martensite the rest being fresh martensite and martensiteaustenite islands the steel sheets combine a tensile strength according to the invention with a low hydrogen amount, thus limiting the risk of hydrogen embrittlement.

[0060]

[0048] In trials 8 and 9, the steel sheets have no risk of hydrogen embrittlement because of the small hydrogen content in the steel. Nevertheless, a microstructure with too many bainite and tempered martensite makes it difficult to achieve strength targets after the galvanizing step.

[0061]

[0049] The microstructure of the steel sheet of trial 10 contains a low amount of bainite. Hydrogen is thus trapped, and the risk of hydrogen embrittlement is increased.

[0062] Table 5 - Weldability properties of the zinc-coated and optionally tempered steel sheet

[0063]

[0050] Spot welding in standard ISO 18278-2 condition was done on the zinc-coated steel sheets, before the optional tempering at T4. Then, for the concerned trials, the tempering step was performed. And finally, a test was done to measure the weld strength.

[0064]

[0051] In the test used, the samples are composed of two sheets of steel in the form of cross welded equivalent. A force is applied so as to break the weld point. This force, known as cross tensile strength (CTS), is expressed in daN. It depends on the diameter of the weld point and the thickness of the metal, that is to say the thickness of the steel and the metallic coating. It makes it possible to calculate the coefficient a which is the ratio of the value of CTS on the product of the diameter of the welded point multiplied by the thickness of the substrate. This coefficient is expressed in daN / mm2.

[0065]

[0052] Weldability properties of the obtained zinc-coated steel and optionally tempered sheet are determined and gathered in the following table:

[0066]

[0053] Even with a content of carbon above or equal to 0.27%C, the steels according to the invention have good spot weldability, with an a value above or equal to 30 daN / mm2. Preferably the a value is above or equal to 40 daN / mm2or even more preferably above or equal to 50 daN / mm2.

[0067]

[0054] In trials 1 and 2, the steel sheet with composition A is subjected to the same process parameters. The steel sheet in trial 2 is submitted to a further tempering step after the welding. The same for trials 4 and 5, the steel sheet with composition B is subjected to the same process parameters. The steel sheet in trial 5 is submitted to a further tempering step after the welding.

[0068] This tempering step after welding leads to an improvement in weld strength, with a higher a value when the tempering is performed. Preferably, the a value of the zinc- coated and tempered steel sheet, when the tempering is done after welding, is above or equal to 50 daN / mm2, more preferably above or equal 60 daN / mm2.

[0069]

[0055] This tempering step can be, for example, a paint baking.

Claims

CLAIMS1 . Zinc coated steel sheet, made of a steel having a composition comprising, by weight percent:C: 0.20 - 0.35 %,Mn: 0.5 - 3.0 %,B: 0.0005 - 0.005%,Si: 0.01 - 1.20%,Cr: 0.01 - 1.5 %, S < 0.010 %, P < 0.020 %, N < 0.012 %, and comprising optionally one or more of the following elements, in weight percentage:Al < 1.0%,Mo < 0.4 %,Ti < 0.050 %,Nb < 0.05 %, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel sheet having a microstructure comprising, in area fraction,- bainite and tempered martensite, the sum being comprised from 25% to 65%- the rest being fresh martensite and martensite-austenite islands.

2. A zinc coated steel sheet according to claim 1 , wherein the steel has a hydrogen content below or equal to 0.3 ppm.

3. A zinc coated steel sheet according to any one of claims 1 and 2, wherein the steel has a tensile strength TS above or equal to 1450MPa.

4. A method for manufacturing a zinc coated steel sheet, comprising the following successive steps: casting a steel to obtain a semi-product, said semi product having a composition according to claim 1 , reheating the semi-product at a temperature comprised between 1150°C and 1300°C,hot rolling the semi-product with a finish hot rolling temperature between 800°C and 1000°C to obtain a hot-rolled steel sheet, cooling the hot rolled steel sheet, coiling the hot rolled steel sheet at a coiling temperature TCOii below or equal to 700°C, cooling the coiled steel sheet optionally heating the steel sheet to a temperature Ti from 500°C to 650°C, and maintaining at said temperature for a holding time ti comprised from 0.1 to 120h before cooling the steel sheet, cold rolling the steel sheet, to obtain a cold rolled steel sheet, heating the cold rolled steel sheet up to a temperature T2 above or equal to Ae3 and maintaining at said temperature for a holding time t2 of 1 to 3600s, cooling the steel sheet to a temperature Tccomprised from (Ms-50°C) to 450°C, maintaining at said Tctemperature for a holding time tcof 3s to 1000s, in order to obtain a heat-treated steel sheet, coating the heat-treated steel sheet with a zinc coating, cooling the steel sheet in order to obtain a zinc coated steel sheet.

5. A method for manufacturing a zinc coated steel sheet, comprising the following successive steps: casting a steel to obtain a semi-product, said semi product having a composition according to claim 1 , reheating the semi-product at a temperature comprised between 1150°C and 1300°C, hot rolling the semi-product with a finish hot rolling temperature between 800°C and 1000°C to obtain a hot-rolled steel sheet, cooling the hot rolled steel sheet, coiling the hot rolled steel sheet at a coiling temperature TCOii below or equal to 700°C, cooling the coiled steel sheet optionally heating the steel sheet to a temperature Ti from 500°C to 650°C, and maintaining at said temperature for a holding time ti comprised from 0.1 to 120h before cooling the steel sheet, cold rolling the steel sheet, to obtain a cold rolled steel sheet, heating the cold rolled steel sheet up to a temperature T2 above or equal to Ae3 and maintaining at said temperature for a holding time t2 of 1 to 3600s,cooling the steel sheet to a temperature Tccomprised from (Ms-50°C) to 450°C, reheating the steel sheet to a temperature T3 higher than Tcand below or equal to 450°C, maintaining at said T3 temperature for a holding time tcof 3s to 1000s, in order to obtain a heat-treated steel sheet, coating the heat-treated steel sheet with a zinc coating, cooling the steel sheet in order to obtain a zinc coated steel sheet.

6. A method according to any one of claims 4 and 5, wherein the zinc coated steel sheet is reheated to a temperature T4 comprised from 100°C to 300°C and maintained at said temperature for a holding time t40f 0.05min to 2400min.

7. A resistance spot weld of two steel parts of the zinc coated steel sheet according to any one of claims 1 to 3 or produced according to any one of claims 4 to 5, said resistance spot weld having an a value of at least 30 daN / mm2.

8. A resistance spot weld of two steel parts of the zinc coated steel sheet according to any one of claims 1 to 3 and produced according to claim 6 , said reheating step to T4 being done after the welding, said resistance spot weld having an a value of at least 50 daN / mm2.

Images