High-strength steel sheet with excellent formability and its production method

Abstract

Summary: Cold rolled and heat treated steel sheet has a composition consisting of ≦0.24% carbon 0.17% ≦, ≦2.2% manganese 1.9% ≦, ≦1.2% aluminum 0.5% ≦, ≦1% silicon 0.5% ≦, ≦0.2% chromium 0.05% ≦, ≦0.2% niobium 0.015% ≦, ≦0.003% sulfur, ≦0.03% phosphorus and optionally containing ≦0.05% titanium 0.005% ≦, ≦0.05% molybdenum 0.001% ≦, the balance of the composition consisting of iron and unavoidable impurities resulting from supersaturation, where Si + Al ≥ 1.3%, the balance including iron and unavoidable impurities Unacceptable due to the microstructure of the tar Coated steel sheet containing, in surface fraction, 10 to 20% retained austenite, said austenite phase having a carbon content of between 0.9 and 1.1%, 40 to 55% polyhedral ferrite, 15 to 40% granular bainite and at least 5% tempered martensite, the sum of martensite and retained austenite being between 20 and 30%.

Description

High-strength steel sheet with excellent formability and its manufacturing method The present invention relates to a steel sheet having excellent mechanical properties suitable for use in the manufacture of automobiles, in particular, the present invention relates to a highly formable sheet with high strength, and a method of manufacturing the same. In recent years, the increasing emphasis on fuel economy and carbon footprint from the perspective of environmental protection has necessitated the reduction of automobile weight; consequently, it is necessary to create steel sheets with high strength, increased elongation, and acceptable mechanical properties. Therefore, automobile steel parts need to satisfy two characteristics that are generally difficult to consider together: high formability and ductility on the one hand, and high tensile strength on the other. Much research and manufacturing effort has been made to reduce the weight of the machine by increasing the strength of the materials. Conversely, increasing the strength of steel sheets reduces the formability, and the creation of materials with both high strength and high formability is needed. Therefore, highly formable sheets such as TRIP steels ("Transformation Induced Plasticity") have been developed. TRIP steels offer a good balance between mechanical strength and formability due to their complex structure that involves progressive transformation with strain. TRIP steels can also contain ferrite, which is a ductile component, and components such as martensite and austenite (MA) islands and bainite. TRIP steels have a high capacity for consolidation, which allows a good distribution of deformations in the event of a collision or even during the forming of the automotive part. It is therefore possible to manufacture parts that are as hard as those made from conventional steels but with improved mechanical properties, which in turn allows the thickness of the parts to be reduced to meet the same performance specifications in terms of mechanical performance. These steels are therefore an effective response to the requirements of reduced weight and increased safety in vehicles.In the field of hot-rolled or cold-rolled steel, this type of steel has applications in, among other things, structural and safety parts for automotive vehicles. Efforts have been made to create steel with high strength and high formability, which has led to a range of high-strength steels, high-formability steels, and methods for producing high-strength, high-formability steel sheets. US9074272 describes steels having the following chemical compositions: 0.1-0.28% C, 1.0-2.0% Si, 1.0-3.0% Mn and the remainder, iron and unavoidable impurities are considered. The microstructure consists of 9-17% retained austenite, 40-65% bainitic ferrite, 30-50% polygonal ferrite and less than 5% martensite. Excellent elongation is attributed to cold rolled steel sheet but the invention described in US9074272 is ineffective in achieving a tensile strength of 900 Mpa which is now required for a number of automotive structural components. US 2015 / 0152533 describes a method for producing a high-strength steel sheet having C: 0.12 −0.18%, Si: 0.05 −0.2%, Mn: 1.9 −2.2%, Al: 0.2 −0.5%, Cr: 0.05 −0.2%, Nb: 0.01 −0.06%, P: ≦0.02%, S: ≦0.003%, N: ≦0.008%, Mo: ≦0.1%, B: ≦0.0007%, Ti: ≦0.01%, Ni: ≦0.1%, Cu: ≦0.1% and, as a remainder, iron and unavoidable impurities. The steel sheet has a microstructure that consists of 50-90% by volume of ferrite, which includes bainitic ferrite, 5-40% by volume of martensite, up to 15% by volume of retained austenite, and up to 10% by volume of other structural components. Although the steel disclosed in US2015 / 0152533 contains a substantial amount of martensite (i.e. up to 40%), this steel is unable to achieve a tensile strength level of 900Mpa. Furthermore, JP 2001 / 254138 describes steels having the following chemical compositions: 0.001-2.0% Al, 0.05-0.3% C, 0.3-2.5% Si, 0.5-3.0% Mn, the remainder consisting of iron and unavoidable impurities. The structure comprises retained austenite in which the mass concentration of carbon is greater than or equal to 1% and the volume fraction is between 3 and 50%, as well as ferrite in an amount of 50 to 97%. The invention cannot be used for the manufacture of steels which are required for specific mechanical strength combined with high ductility to create a complex structural part for an automobile vehicle. In addition, EP2765212 processes a high-strength steel sheet having excellent formability and tensile flangeability, which has a microstructure in the form of a surface proportion of martensite of 5-70%, a surface proportion of retained austenite of 5-40%, and a surface proportion of bainitic ferrite in upper bainite of 5% or more, and a total of 40% or more, 25% or more of martensite is tempered martensite, and a surface proportion of polyhedral ferrite is above 10% and below 50%. Therefore, taking advantage of the publications mentioned above, the object of this invention is to provide a steel sheet that makes it possible to achieve greater weight reduction with a capacity that corresponds to the current automobile manufacturing for the manufacture of complex parts and members. The present invention aims to solve these problems by providing cold-rolled steel sheets that simultaneously have: -The ultimate tensile strength TS is greater than or equal to 980 Mpa and preferably above 1050 Mpa, or even more than 1100 Mpa, -Has a yield strength above 550Mpa, -Have a submission ratio equal to or above 0.60, -Total TE length increase greater than or equal to 17% and preferably above 19%, -Have a cavity expansion ratio greater than or equal to 18% (measured according to ISO 16630:2009). Preferably, such steels have a good formability, especially for rolling, and a good weldability and coating capability. Another object of the present invention is to produce steel that is excellent in resisting liquid metal embrittlement cracking. Another object of the present invention is also to provide a method for manufacturing steels that are compatible with conventional industrial applications while not being too sensitive to small variations in manufacturing parameters. Figure 1 is a micrograph showing the microstructure of the steel of the present invention. The martensite and tempered austenite appear as pale components, the remainder being ferrite and granular bainite. Figure 2A shows the homogeneous distribution of tempered martensite in the steel sheet of this invention, while Figure 2B shows the heterogeneous distribution of martensite in the reference steel sheet. The steel sheet according to this invention presents a specific composition which will be explained in detail. Carbon in the steel of the present invention is present in an amount of from 0.17% to 0.24%. Carbon plays an important role in the formation of the microstructure and in the strength and ductility through the TRIP effect. It is not possible to obtain a TRIP effect when the carbon is less than 0.17%. Above 0.24%, weldability decreases. The carbon content is advantageously between 0.20 and 0.24%, to obtain simultaneously high strength and high elongation. Manganese is added to the present steel in an amount of 1.9% to 2.2%. Manganese is an element that provides hardening by replacing solid solution in ferrite. A minimum amount of 1.9% by weight is required to obtain the desired tensile strength. However, above 2.2%, manganese slows down the formation of bainite and increases the formation of austenite with a reduced amount of carbon that ultimately transforms into martensite relative to retained austenite, which is detrimental to the required properties. Silicon is added to the steel of the present invention in an amount of 0.5% to 1%. Silicon plays an important role in the formation of the microstructure by slowing down the precipitation of carbides during the equalization stage after the initial cooling, which allows the carbon to be concentrated in the austenite to stabilize it. Silicon plays an effective role in combination with the role of aluminum, the best results of which, in relation to the specific characteristics, were obtained at levels above 0.5%. However, the addition of silicon in an amount of more than 1% has a detrimental effect on the hot dip galvanizability by promoting the formation of oxides that adhere to the surface of the product, which reduces the weldability. It also leads to embrittlement of the liquid metal by the penetration of Zn into the austenitic grain boundaries during spot welding. A value less than or equal to 1% simultaneously offers good weldability and also good weldability. The silicon content is preferably between 0.7 and 0.9% to limit the formation of brittle martensite instead of bainite. Aluminum plays an important role in this invention by significantly slowing down the precipitation of carbides and stabilizing the retained austenite. This effect is achieved when the aluminum content is between 0.5% and 1.2%. The aluminum content is preferably less than or equal to 0.9% and greater than or equal to 0.7%. It is also generally believed that high levels of Al increase wear of hard materials and the risk of nozzle pitting during upstream casting of rolled steel. Aluminum also segregates negatively and can lead to macrosegregations. In high amounts, aluminum reduces hot formability and increases the risk of defects appearing during continuous casting. Without careful control of the casting conditions, micro- and macrosegregation defects will eventually lead to a central segregation in the annealed steel sheet. This central band will be harder than the surrounding matrix and will adversely affect the formability of the material. In addition to the individual limitations explained above, the sum of aluminum and silicon should be above 1.3% and preferably above 1.4%, since both elements contribute synergistically to the stabilization of retained austenite, which significantly slows down the precipitation of carbides during the annealing cycle, and very specifically during the bainitic transformation. It makes it possible to obtain an enrichment of the austenite with carbon, which leads to its stability at room temperature in the steel sheet. Furthermore, the inventors have found that when Si / 10 > 0.30% – C (Si and C are expressed as weight percentages), silicon is detrimental to spot welding of coated sheets, especially galvanized or electro-galvanized sheets, due to LME (liquid metal embrittlement phenomenon). The occurrence of LME causes grain boundary cracks in the heat affected zone and in the weld metal of welded joints. Therefore, (C + Si / 10) should be kept less than or equal to 0.30%, especially if the sheet is to be coated. They also found that for the occurrence of LME, for the compositional range of interest, the Al content must be greater than or equal to 6(C+Mn / 10) – 2.5%. Chromium is added to the steel of the present invention in an amount of 0.05% to 0.2%. Chromium, like manganese, increases the hardenability by promoting the formation of martensite. When the chromium content is greater than 0.05%, it is beneficial to achieve the required tensile strength, however, when the chromium content is greater than 0.2%, the formation of bainite is delayed, so that the austenite is not sufficiently enriched with carbon during the equalization step; in fact, this austenite is more or less completely transformed into martensite during cooling to ambient temperature, and the elongation is very low. Therefore, the chromium content is between 0.05 and 0.2%. Niobium is added to the steel of the present invention in an amount of 0.015 to 0.03 to initiate the formation of carbo-nitrides to contribute to strength by accelerating hardening. Because niobium delays recrystallization during heating, the microstructure formed at the end of the holding temperature and as a result after complete annealing is linear, which leads to hardening of the product. However, when the niobium content is above 0.03%, more carbo-nitrides are formed and tend to reduce the ductility of the steel. Titanium is an optional element that may be added to the steel of the present invention in an amount of 0.005% to 0.05%. In the form of niobium, it precipitates to form carbo-nitrides and participates in hardening. But it also participates in the formation of large TiN that appears during the solidification of the casting. The amount of titanium is therefore limited to 0.05% to avoid coarse TiN that is detrimental to the expansion of pores. If the amount of titanium is added in an amount below 0.005%, it will have no effect on the steel of the present invention. Molybdenum is an optional element that may be added to the steel of the present invention in an amount of 0.001% to 0.05%. Molybdenum can play an effective role in increasing hardenability, which delays the formation of bainite and prevents the precipitation of carbides in bainite. However, the addition of molybdenum greatly increases the cost of adding alloying elements, so that its amount is limited to 0.05% for economic reasons. The sulfur content in the present invention should be kept as low as possible; hence, the sulfur content is less than or equal to 0.004% in the present invention. A sulfur content of 0.004% or more reduces the ductility due to the increased presence of sulfides such as MnS (manganese sulfides), which reduce the workability of the steel, and is also a source of crack initiation. Phosphorus can be present in the steel of the present invention in amounts up to 0.03%, phosphorus is an element that hardens in solid solution but significantly reduces the suitability for spot welding and hot formability. For these reasons, its amount should be limited to 0.03% to make it suitable for spot welding and hot formability. The steel sheet of the present invention exhibits a special microstructure consisting of several phases, the amount of which is presented in surface fractions. The polyhedral ferrite component contributes to the improved elongation in the steel of the present invention, ensuring an increase in elongation and a pore expansion ratio at the required levels. Polyhedral ferrite is a soft and inherently ductile component. It can be distinguished from the normal ferrite formed during the annealing step because it has a low solid solution carbon content and a very low dislocation density. Polyhedral ferrite should be present in a minimum amount of 40% and up to a maximum level of 55%. Polyhedral ferrite provides the elongation in the present invention due to its softness compared to the other hard phases present such as tempered martensite and due to the very limited amount of carbon present in polyhedral ferrite which may be 0.005%. In addition, the low dislocation density also contributes to the pore expansion ratio. This polyhedral ferrite is essentially formed during heating and holding at a temperature corresponding to an intracritical annealing. Some amount of normal ferrite may be formed during cooling, but due to the manganese content, the amount of normal ferrite that appears during the cooling stage is always less than 5%. The granular bainite present in the present invention is distinguished from the conventional bainite structure because the granular bainite of the present invention has a very low density of carbides. A low density of carbides here means less than or equal to 100 carbides per unit area of ​​100µm2. Because the dislocation density is high (about 1015 / m-2), this granular bainite provides the high strength of the steel of the present invention, unlike polyhedral ferrite. The amount of granular bainite is from 15 to 40%. Retained austenite is present as a component in an amount between 10 and 20% and is the main component to ensure the TRIP effect. The retained austenite of the present invention has a carbon content of 0.9 to 1.1%, which plays an important role in stabilizing the austenite at room temperature and ensuring the TRIP effect that provides the present invention with good formability. In addition, the carbon-rich retained austenite also contributes to the formation of granular bainite when the solubility of carbon in austenite is very high, which slows down the formation of carbides in bainite. In a preferred embodiment, the average grain size of the retained austenite is less than 2 µm. The retained austenite is measured by a magnetic method called sigmametry, which involves measuring the magnetic moment of the steel before and after the heat treatment that destabilizes the austenite, which is paramagnetic, unlike the other phases, which are ferromagnetic. The steel of this invention also contains at least 5% tempered martensite, which is a component consisting of fine grains elongated in one direction within each grain originating from a primary austenite grain, in which fine iron carbides are interspersed between the grains in the same direction. <111> precipitates. This martensite tempering allows for an increase in yield strength due to the reduction of the hard gap between martensite and ferrite or bainite and increases the pore expansion ratio due to the reduction of martensite. The total amount of tempered martensite and retained austenite is between 20 and 30%, and preferably between 25 and 30%. The tempered martensite and austenite can be present either in the form of martensite-austenite islands or in the form of individual microstructures. The present steel does not contain any annealed martensite because annealed martensite is a hard phase and thereby reduces the yield strength of the steel and will also reduce the formability of the steel of the present invention. In a preferred embodiment of the present invention, the homogeneity of the distribution of the amount of tempered martensite is determined as follows: The tempered martensite fraction (TM) measured on each 50x50 µm² surface of said steel sheet is compared with the average fraction (TM*). A homogeneous tempered martensite distribution is defined if |(TM)-(TM*)| ≤ 1.5%. Such a homogeneous redistribution improves the cavity expansion ratio. The steel sheet according to the invention can be produced by any suitable process. However, it is preferred to use the process described below. Casting of a semi-finished product can be done in the form of ingots or in the form of thin plates or strips, i.e. with a thickness range from approximately 220 mm for plates to a few tens of mm for strips or thin plates. For the sake of simplicity, the following description will focus on the plates as a semi-finished product. A plate having the chemical composition described above is made by continuous casting, and is prepared for further processing as a creative manufacturing method. Therefore, the plate may be used at an elevated temperature during continuous casting or may be initially cooled to room temperature and then reheated. The temperature of the plate subjected to hot rolling is preferably above the Ac3 point and at least above 1000°C and should be below 1280°C. The temperatures mentioned here are specified to ensure that all points in the plate reach the austenite range. In the case where the plate temperature is below 1000°C, it imposes a high load on the rolling machine, and in addition, the temperature of the steel may drop to a ferrite transformation temperature during rolling. Therefore, to ensure that rolling is complete in the austenite region, reheating should be carried out above 1000°C. Furthermore, the temperature should not be above 1280°C to avoid the opposite growth of the austenitic grain leading to coarse ferrite grains which reduce the capacity of these grains to recrystallize during hot rolling. Furthermore, temperatures above 1280°C increase the risk of the formation of thick oxide layers which are detrimental during hot rolling. The final rolling temperature should be above 850°C. Having a final rolling temperature above the Ar3 point is preferred to ensure that the steel subjected to hot rolling is in the complete austenite zone. The hot rolled steel sheet obtained in this way is then cooled at a cooling rate of 35 to 55° C / s to a coiling temperature equal to or less than 580° C to obtain the microstructure required by the present invention, since this range of cooling rates is conducive to the formation of bainite. The cooling rate should not be higher than 55° C / s to avoid excessive martensite formation. The coiling temperature should be below 580° C, since above this temperature there is a risk of increased microsegregation and intragranular oxidation. The preferred coiling temperature for the hot rolled steel sheet of the present invention is between 450 and 550° C. Finally, the hot-rolled steel sheet is allowed to cool to room temperature at a cooling rate that is preferably not greater than 125°C / h. After that, pickling is performed on the hot rolled steel sheet to remove scale. Hot rolled steels are cold rolled with a thickness reduction usually between 30 and 90%. The resulting cold-rolled steel sheet, obtained by the cold rolling process, is subjected to intra-critical annealing and subsequent heat treatment processes to provide the required mechanical properties and microstructure in the steel of the present invention. The cold rolled steel sheet is continuously annealed, at a heating rate of 1 to 20°C / s and preferably greater than 2°C / s, to a dip temperature between Ac1 and Ac3 and preferably between 780 and 950°C designed to ensure an austenite ratio of 60:40 to 35:65. The dip is preferably carried out for a duration of more than 10 seconds and should be less than or equal to 600 seconds. The steel is then cooled at a rate greater than 25°C / s to the bainite transformation temperature range of 440 to 480°C, preferably greater than or equal to 30°C / s. While not wishing to be bound by theory, the inventors believe that the homogeneity of martensite formation is significantly due to this high cooling rate after annealing. The steel sheet is then held at this temperature for 20 to 250 seconds, and preferably for 30 to 100 seconds to initiate bainite formation. Holding the cold rolled steel sheet for less than 20 seconds will result in a very low amount of bainite and not enough austenite to result in a retained austenite content of less than 10%. Above 250 seconds, it will result in the precipitation of carbides in the bainite, which will free the austenite in carbon before final cooling. This holding is done between 440 and 480°C to form granular bainite and to facilitate enrichment of the austenite in carbon. Hot dip galvanizing (GI) is then carried out by immersing the product in a bath of zinc or zinc alloy, the temperature of which can be between 440 and 475°C, and then allowing the GI product to cool to room temperature at a cooling rate of 1 to 20°C / s, and preferably between 5 and 15°C / s, to obtain retained austenite and limit the amount of martensite. The galvanized steel sheet is then subjected to an annealing treatment step. During this annealing step, the galvanized steel sheet is heated to a temperature between 170 and 350°C, preferably between 170 and 250°C, for 12 to 250 hours, preferably 12 to 30 hours, and then cooled to room temperature. This is done to effectively form fresh martensite. Examples The tests, examples, illustrative examples and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, and illustrate the useful features of the present invention and explain the importance of the process parameters selected by the inventors after extensive testing and further define the characteristics that can be achieved by the steel of the present invention. The steel sheet compositions of the test specimens are collected in Table 1, where the steel sheets were produced according to the process parameters collected respectively in Table 2. Table 3 shows the obtained microstructures and Table 4 shows the results of the evaluations of the used characteristics. It is worth noting that, due to differences in measurement methods, the values ​​of the cavity expansion ratio HER according to the ISO standard are very different and are not comparable with the values ​​of the cavity expansion ratio λ according to JFS T 1001 (Japan Iron and Steel Federation standard). The tensile strength TS and the total elongation TE were measured according to the ISO standard, ISO 6892-1, published in October 2009. Due to differences in measurement methods, especially due to differences in the geometry of the sample used, the values ​​of the total elongation TE measured according to the ISO standard are very different, especially lower, than the values ​​of the total elongation measured according to the JIS Z 2201-05 standard. Table 1-Steel compositions Table 1 shows the steels with compositions expressed as weight percent. Steel compositions I1 and I6 are used to make steels according to this invention, this table also shows the compositions of the alloy steels, which are indicated in the table from R1 to 9. C Mn Al Si Cr Nb NSP Si+Al C+Si / 10 I1 0.224 2.210 0.779 0.710 0.053 0.019 0.0023 0.0037 0.011 1.489 0.30 I2 0.221 2.050 0.775 0.724 0.056 0.020 0.0023 0.0034 0.012 1.499 0.29 I3 0.193 2.010 0.785 0.720 0.107 0.020 0.0024 0.0019 0.012 1.505 0.27 I4 0.206 2.100 0.782 0.715 0.106 0.020 0.0025 0.0020 0.010 1.497 0.28 I5 0.205 2.190 0.782 0.718 0.106 0.020 0.0025 0.0020 0.011 1.500 0.28 I6 0.220 2.190 0.781 0.719 0.106 0.020 0.0025 0.0019 0.012 1.500 0.29 R1 0.190 2.010 0.023 1.170 0.01 0.020 0.0027 0.0025 0.011 1.193 0.31 R2 0.192 2.000 0.019 1.470 0.011 0.019 0.0280 0.0026 0.012 1.489 0.34 R3 0.188 2.300 0.770 0.676 0.024 0.019 0.0024 0.0018 0.012 1.446 0.26 R4 0.197 1.610 0.761 0.680 0.311 0.020 0.0026 0.0020 0.011 1.441 0.27 R5 0.241 1.680 0.787 0.727 0.304 0.020 0.0027 0.0030 0.015 1.514 0.31 R6 0.222 2.050 0.768 0.730 0.022 0.002 0.0026 0.0033 0.012 1.498 0.30 R7 0.220 1.840 0.762 0.731 0.011 0.030 0.0026 0.0031 0.013 1.493 0.29 R8 0.221 2.040 0.758 0.729 0.256 0.030 0.0028 0.0034 0.012 1.487 0.29 R9 0.224 2.040 0.763 0.733 0.025 0.030 0.0026 0.0032 0.013 1.496 0.30. I = In accordance with this invention; R = Reference; Underlined values: Not in accordance with this invention. Table 2- Process parameters Table 2 here shows the details of the annealing process parameters performed on the steel samples shown in Table 1. Table 1 also shows the tabulation of the bainite transformation temperatures of the innovative steel and the reference steel. The calculations of the bainite transformation temperatures are done using the following formula: Bs=839-(86*[Mn]+23*[Si]+67*[Cr]+33*[Ni]+75*[Mo])-270*(1-EXP(-1,33*[C])) Ac1 is calculated using the formula published in "“Darstillung der Umwandlungen für technische Anwendungen und Möglichkeiten ihrer Beeinflussung , HP Hougardy, Werkstoffkunde Stahl Volume 1,198-231, Verlag Stahleisen, Düsseldorf, 1984”": Ac1 = 739 - 22*C – 7*Mn +2*Si +14*Cr+13*Mo- 13*Ni. In this formula, Ac1 is in Celsius, and C, Mn, Si Cr, Mo and Ni are the weight percentages of C, Mn, Si, Cr, Mo and Ni in steel. Ac3 is calculated using Thermo-Calc® software. The steel samples were heated to temperatures between 1000°C and 1280°C and then subjected to hot rolling with a final temperature above 850°C, after which they were coiled at a temperature below 580°C. The hot rolled coils were then cold rolled with a thickness reduction of between 30 and 80%. These cold rolled steel sheets were subjected to the heat treatment described below. They were then hot dipped galvanized in a zinc bath at 460°C and batch annealed for 24 hours. Table 2-Hot and cold rolling stages Reheat temperature (°C) Final temperature (°C) Cooling rate (°C / s) Coiling temperature (°C) Cold rolling reduction (%) I1 1200 900 35 540 50 I2 1180 910 37 535 55 I3 1180 890 40 545 54 I4 1200 888 42 550 60 I5 1210 875 38 520 65 I6 1200 915 39 525 58 R1 1250 935 26 530 50 R2 1250 907 27 535 52 R3 1200 843 35 545 62 R4 1200 875 45 520 60 R5 1230 850 43 550 55 R6 1220 890 38 535 58 R7 1220 895 32 540 51 R8 1250 892 37 545 53 R9 1250 905 45 535 65 Table 2 - Continuous annealing stage Ac1 (°C) Ac3 (°C) Heating rate (°C / s) T immersion (°C) Immersion time (sec) Cooling rate (°C / s) T cooling (°C) Holding time (sec) I1 720 895 4.4 820 65 45 475 35 I2 722 905 4.4 820 65 45 475 35 I3 723 925 4.4 780 65 45 475 35 I4 722 915 4.4 820 65 45 475 35 I5 722 905 4.4 820 65 45 475 35 I6 721 900 2.8 820 100 25 475 53 R1 723 830 4.1 820 72 22 475 38 R2 723 840 5.2 820 80 17 475 35 R3 720 905 5.2 820 80 17 475 49 R4 729 925 5.2 820 80 17 475 49 R5 727 915 5.2 820 80 17 475 49 R6 721 905 5.2 820 80 17 475 49 R7 722 910 5.2 820 80 17 475 49 R8 724 905 4.4 820 65 45 475 49 R9 721 900 4.1 820 72 22 475 49 Table 2 - Batch annealing stage Heating rate (°C / s) Immersion temperature (°C) Immersion time (hours) I1 4 300 24 I2 4 210 24 I3 4 210 24 I4 4 210 24 I5 4 210 24 I6 4 210 24 R1 - - - R2 - - - R3 - - - R4 - - - R5 - - - R6 - - - R7 - - - R8 4 210 24 R9 4 210 24 Table 3- Microstructures Table 3 shows the results of tests performed according to standards in various microscopes such as scanning electron microscopy to determine the microstructural composition of both the innovative and reference sheets. The results are expressed in area percent, except for the amount of retained austenitic carbon which is expressed as weight percent. It was observed that all of the inventive examples had a homogeneous martensite redistribution, while all of the comparative examples had a heterogeneous redistribution. Retained austenite Carbon in retained austenite Polyhedral ferrite Bainite Quenched martensite Unquenched martensite I1 14 1.07 45 27 14 0 I2 17 0.96 47 31 5 0 I3 14 1.00 55 20 11 0 I4 15 0.98 50 25 11 0 I5 12 1.01 48 27 13 0 I6 12 0.99 45 30 13 0 R1 13 1.00 20 55 0 12 R2 14 0.92 30 40 0 ​​16 R3 8 0.96 56 19 0 17 R4 11 1.13 60 22 0 7 R5 16 1 60 10 0 14 R6 17 0.98 65 15 0 3 R7 17 1.04 70 10 0 3 R8 12 0.95 45 20 23 0 R9 18 0.92 50 29 3 0 I = In accordance with this invention; R = Reference; Underlined values: Not in accordance with this invention Table 4- Mechanical specifications Table 4 gives examples of the mechanical properties of the innovative steel and the reference steels. The tensile test was carried out in accordance with the NF EN ISO 6892-1 standard. The hole expansion ratio was measured in accordance with the ISO16630:2009 standard, in which the sample was deformed by 10 mm of perforation. After deformation and crack initiation, the hole diameter was measured, and the TER% was calculated = 100*(Df-Di) / Di. The results of various mechanical tests performed in accordance with the standards are tabulated here. Tensile strength (MPa) YS (MPa) YS / TS Overall elongation (%) Cavity expansion ratio (%) I1 1000 729 0.72 17 23 I2 990 633 0.64 18.8 21 I3 1065 735 0.69 17.3 19 I4 1040 669 0.64 17.9 22 I5 1078 723 0.67 17.1 18 I6 1023 641 0.63 17.4 23 R1 957 459 0.48 19.4 22 R2 1008 415 0.40 15.9 13 R3 1097 464 0.42 12.8 nd R4 913 353 0.39 15.8 nd R5 1010 373 0.37 14.9 nd R6 915 398 0.43 19.4 nd R7 896 434 0.48 17.2 nd R8 1251 526 0.42 9.1 nd R9 1061 469 0.44 17.3 10 I = In accordance with this invention; R = Reference; Underlined values: Not in accordance with this invention; nd: Not determined. In relation to spot weldability, steels according to the invention are less sensitive to LME when the composition is such that C + Si / 10 ≤ 0.30%. This means that such steels are likely to produce structures comprising resistant spot welds, such as car bodies, for which the probability of the number of cracks in the resistant spot welds is such that the average value is less than 5 cracks per resistant spot weld and the probability of having less than 10 cracks is 98%. In particular, a welded structure, comprising resistance spot welding of at least two steel sheets, can be produced by producing a primary steel sheet by a method according to the invention, the primary sheet being such that + Si / 10 ≤ 0.30% C and Al ≥ 6(C + Mn / 10) – 2.5% and being coated with Zn or a Zn alloy, providing a secondary steel sheet having a composition such that C + Si / 10 ≤ 0.30% and Al ≥ 6(C + Mn / 10) – 2.5%, and resistance spot welding the primary steel sheet to the secondary steel sheet. The secondary steel sheet may, for example, be produced by a method according to the invention, and be coated with Zn or a Zn alloy. Therefore, the welded structure has a low LME sensitivity. For example, for such a welded structure containing at least ten resistant spot welds, the average number of cracks per resistant spot weld is less than 5. Steel sheets optionally welded by resistance spot welding according to the present invention are advantageously used for the manufacture of structural parts in motor vehicles because they offer a high ductility during the manufacturing process and absorb high energy in the event of a crash. Resistance spot welds according to the present invention are also advantageously used for the manufacture of structural parts in motor vehicles because ultimately the initiation and propagation of cracks formed in the welded areas is greatly reduced. Claims 1-Coated steel sheet, with a composition including the following elements, expressed as a percentage by weight: ≦ 0.24% Carbon ≦ 0.17% ≦ 2.2% Manganese 1.9% ≦ ≦ 1% Silicon ≦ 0.5% ≦ 1.2% Aluminum ≦ 0.5% Where Si + Al ≥ 1.3% ≦ 0.2% Chromium ≦ 0.05% ≦ 0.03% Niobium 0.015% ≦ ≦ 0.004% Sulfur ≦ 0.03% Phosphorus and may contain one or more of the following optional elements: ≦ 0.05% Titanium 0.005% ≦ ≦ 0.05% Molybdenum 0.001% ≦ The residue consists of iron and unavoidable impurities resulting from smelting, the microstructure of said coated steel sheet comprises, in the surface fraction, 10 to 20% retained austenite, said austenite phase having a carbon content between 0.9 and 1.1%, 40 to 55% polyhedral ferrite, 15 to 40% granular bainite and at least 5% tempered martensite, the sum of tempered martensite and retained austenite being between 20 and 30%. 2- Steel sheet according to claim 1, wherein the composition comprises, expressed as a weight percentage, 0.7%≦Si≦0.9%. 3. Steel sheet according to claim 1 or 2, wherein the composition comprises, expressed as a weight percentage, 0.7%≦Al≦0.9%. 4- Steel sheet according to claim 1 or 2, wherein the sum of the amounts of silicon and aluminum is greater than 1.4%. 5- Steel sheet according to any one of claims 1 to 4, wherein the carbon and silicon contents are such that C + Si / 10 ≤ 0.30%. 6- Steel sheet according to any one of claims 1 to 5, wherein the amounts of aluminum, carbon and manganese are such that Al ≥ 6(C+Mn / 10) - 2.5%. 7. Steel sheet according to any one of claims 1 to 6, wherein the sum of retained austenite and tempered martensite is between 25% and 30%. 8. A steel sheet according to any one of claims 1 to 7, having an average tempered martensite fraction (TM*) and a tempered martensite fraction (TM) measured per 50x50 µm² surface in said steel sheet, such that: |(TM)-(TM*)| ≤ 1.5%. 9- Steel sheet according to any one of claims 1 to 8, having a tensile strength greater than or equal to 980 Mpa, a uniform elongation greater than or equal to 17% and a cavity expansion ratio equal to or greater than 18%, wherein the cavity expansion ratio is measured in accordance with ISO standard, 16630:2009. 10. Steel sheet according to claim 9, wherein the tensile strength is between 1000 Mpa and 1100 Mpa and wherein the void expansion ratio is between 18% and 23%. 11- Steel sheet according to any one of claims 9 to 10, wherein the yield strength is above 550 Mpa and the ratio of yield strength to tensile strength is 0.60 or more. 12. Steel sheet according to any one of claims 1 to 11, wherein said steel sheet is hot-dip galvanized. 13-The method of manufacturing coated steel sheet includes the following sequential steps> -Preparing a semi-finished product whose composition corresponds to any one of claims 1 to 6 -Reheating the semi-finished product to a temperature between 1000°C and 1280°C; -Nuro is a semi-finished product completely in the austenitic range, where the final hot rolling temperature must be greater than or equal to 850°C to obtain a hot-rolled steel sheet. -cooling the hot-rolled steel sheet at a cooling rate of 35 to 55°C / s to a coiling temperature of less than or equal to 580°C; and coiling said hot-rolled sheet; -Cooling the said hot rolled sheet to room temperature, -Salting of the said hot rolled steel sheet -Cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; -then continuously annealing said cold rolled steel sheet at a heating rate of 1 to 20°C / s to a water immersion temperature between Ac1 and Ac3 over a time period of less than 600 seconds, -Then cooling the sheet at a rate greater than 25°C / s to a temperature between 400 and 480°C, and holding the cold-rolled steel sheet for a period of 20 to 250 seconds, -Coating cold-rolled steel sheet by hot immersion in a zinc or zinc alloy bath; -Cooling the cold-rolled steel sheet to room temperature; -Then batch annealing of cold rolled steel sheet at a rate between 1°C / s to 20°C / s to a water immersion temperature of 170 to 350°C for 12 to 250 hours, then cooling the sheet to room temperature. 14-A method for producing a coated steel sheet according to claim 13, wherein said coiling temperature is lower than the bainite transformation start temperature Bs. 15- A method of producing a coated steel sheet according to claim 13 or 14, wherein the immersion temperature is between 780°C and 900°C, the immersion being carried out for 10 to 600 seconds. 16. A method of producing a coated steel sheet according to any one of claims 13 to 15, wherein the sheet is cooled to a temperature between 400 and 480°C at a cooling rate greater than 30°C / s after continuous annealing. 17-A method for producing a coated steel sheet according to claim 16, wherein the steel sheet is cooled at a cooling rate of less than 20°C / s after being coated in a zinc or zinc alloy bath. 18. A method of producing a coated steel sheet according to any one of claims 13 to 17, wherein the steel sheet is batch annealed at a temperature between 170°C and 250°C for 12 to 30 hours. 19. A steel sheet according to any one of claims 1 to 12, wherein said steel sheet does not have anhydrous martensite. Figure 1 Figure 2 Figure 2A Figure 2B Summary The cold-rolled and heat-treated steel sheet has a composition consisting of ≦0.24% carbon 0.17% ≦, ≦2.2% manganese 1.9% ≦, ≦1.2% aluminum 0.5% ≦, ≦1% silicon 0.5% ≦, ≦0.2% chromium 0.05% ≦, ≦0.03% niobium 0.015% ≦, ≦0.003% sulfur, ≦0.03% phosphorus and optionally containing ≦0.05% titanium 0.005% ≦, ≦0.05% molybdenum 0.001% ≦, the balance of the composition consisting of iron and unavoidable impurities resulting from superposition, where Si + Al ≥ 1.3%, the balance comprising iron and unavoidable impurities resulting from, microstructure of the coated steel sheet which contains, in the surface fraction, 10 to 20% retained austenite, said austenite phase having a carbon content between 0.9 and 1.1%, 40 to 55% polyhedral ferrite, 15 to 40% granular bainite and at least 5% tempered martensite, the sum of martensite and retained austenite being between 20 and 30%. Shape: None

Claims

Claims 1- A coated steel sheet, having a composition comprising the following elements, expressed as a percentage by weight: ≦ 0.24% carbon 0.17% ≦ ≦ 2.2% manganese 1.9% ≦ ≦ 1% silicon 0.5% ≦ ≦ 1.2% aluminum 0.5% ≦ wherein Si + Al ≥ 1.3% ≦ 0.2% chromium 0.05% ≦ ≦ 0.2% niobium 0.015% ≦ ≦ 0.004% sulfur ≦ 0.03% phosphorus and possibly containing one or more of the following optional elements ≦ 0.05% titanium 0.005% ≦ ≦ 0.05% molybdenum 0.001% ≦ the remainder comprising iron and unavoidable impurities resulting from smelting, the microstructure of said coated steel sheet comprising, in fraction Surface, 10 to 20% retained austenite, said austenite phase has a carbon content between 0.9 and 1.1%, 40 to 55% polyhedral ferrite, 15 to 40% granular bainite and at least 5% tempered martensite, the total of tempered martensite and retained austenite is between 20 and 30%. 2- Steel sheet according to claim 1, wherein the composition comprises, expressed as a weight percentage, 0.7%≦Si≦0.9%.

3. Steel sheet according to claim 1 or 2, wherein the composition comprises, expressed as a weight percentage, 7%≦Al≦0.9%. 4- Steel sheet according to claim 1 or 2, wherein the sum of the amounts of silicon and aluminum is greater than 1.4%. 5- Steel sheet according to any one of claims 1 to 4, wherein the carbon and silicon contents are such that C + Si / 10 ≤ 0.30%. 6- Steel sheet according to any one of claims 1 to 5, wherein the amounts of aluminum, carbon and manganese are such that Al ≥ 6(C+Mn / 10) - 2.5%.

7. Steel sheet according to any one of claims 1 to 6, wherein the sum of retained austenite and tempered martensite is between 25% and 30%.

8. A steel sheet according to any one of claims 1 to 7, having an average tempered martensite fraction (TM*) and a tempered martensite fraction (TM) measured per 50x50 µm² surface in said steel sheet, such that: |(TM)-(TM*)| ≤ 1.5%. 9- Steel sheet according to any one of claims 1 to 8, having a tensile strength greater than or equal to 980 Mpa, a uniform elongation greater than or equal to 17% and a cavity expansion ratio equal to or greater than 18%, wherein the cavity expansion ratio is measured in accordance with ISO standard, 16630:2009.

10. Steel sheet according to claim 9, wherein the tensile strength is between 1000 Mpa and 1100 Mpa and wherein the void expansion ratio is between 18% and 23%. 11- Steel sheet according to any one of claims 9 to 10, wherein the yield strength is above 550 Mpa and the ratio of yield strength to tensile strength is 0.60 or more. 12- Steel sheet according to any one of claims 1 to 11, wherein said steel sheet is hot-dip galvanized. 13- A method of manufacturing a coated steel sheet comprising the following sequential steps: - providing a semi-finished product having a composition according to any one of claims 1 to 6; - reheating the semi-finished product to a temperature between 1000°C and 1280°C; - further annealing the semi-finished product completely in the austenitic range, which final hot-rolling temperature must be greater than or equal to 850°C, to obtain a hot-rolled steel sheet.-cooling the hot-rolled steel sheet at a cooling rate of 35 to 55°C / s to a coiling temperature of less than or equal to 580°C; and coiling said hot-rolled sheet; -cooling said hot-rolled sheet to room temperature, -salting said hot-rolled steel sheet; -rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; -then continuously annealing said cold-rolled steel sheet at a heating rate of 1 to 20°C / s to a water immersion temperature between Ac1 and Ac3 during a time of less than 600 seconds, -then cooling the sheet at a rate of more than 25°C / s to a temperature between 400 and 480°C, and holding the cold-rolled steel sheet for a period of 20 to 250 seconds, -coating the cold-rolled steel sheet by hot-dip dipping in a zinc or zinc alloy bath; -Cooling the cold-rolled steel sheet to room temperature; -Then batch annealing the cold-rolled steel sheet at a rate between 1°C / s to 20°C / s to a water immersion temperature of 170 to 350°C for 12 to 250 hours, then cooling the sheet to room temperature. 14-A method for producing a coated steel sheet according to claim 13, wherein said coiling temperature is lower than the bainite transformation start temperature Bs. 15- A method of producing a coated steel sheet according to claim 13 or 14, wherein the immersion temperature is between 780°C and 900°C, the immersion being carried out for 10 to 600 seconds.

16. A method of producing a coated steel sheet according to any one of claims 13 to 15, wherein the sheet is cooled to a temperature between 400 and 480°C at a cooling rate greater than 30°C / s after continuous annealing. 17-A method for producing a coated steel sheet according to claim 16, wherein the steel sheet is cooled at a cooling rate of less than 20°C / s after being coated in a zinc or zinc alloy bath.

18. A method of producing a coated steel sheet according to any one of claims 13 to 17, wherein the steel sheet is batch annealed at a temperature between 170°C and 250°C for 12 to 30 hours.

19. A steel sheet according to any one of claims 1 to 12, wherein said steel sheet does not have anhydrous martensite.

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