The present invention deals with a low density steel sheet and in particular a duplex microstructure. The steel sheet according to the invention is particularly well suited for the manufacture of inner or outer panels for vehicles such as land motor vehicles.

Environmental restrictions are forcing automakers to continuously reduce the CO2 emissions of their vehicles. To do that, automakers have several options, whereby their principal options are to reduce the weight of the vehicles or to improve the efficiency of their engine systems. Advances are frequently achieved by a combination of the two approaches. This invention relates to the first option, namely the reduction of the weight of the motor vehicles. In this very specific field, there is a two-track alternative:

The first track consists of reducing the thicknesses of the steels while increasing their levels of mechanical strength. Unfortunately, this solution has its limits on account of a prohibitive decrease in the rigidity of certain automotive parts and the appearance of acoustical problems that create uncomfortable conditions for the passenger, not to mention the unavoidable loss of ductility associated with the increase in mechanical strength.

The second track consists of reducing the density of the steels by alloying them with other, lighter metals. Among these alloys, the low-density ones have attractive mechanical and physical properties while making it possible to significantly reduce the weight.

In particular, EP3421629 is patent that claims for a high strength cold-rolled and heat-treated steel strip, sheet, blank or hot formed product having a bimodal microstructure comprising the steps of producing and casting a melt into a slab or cast strip having the following composition;0.05 to 0.50 wt% C;0.50 to 8.0 wt.% Mn; 0.05 - 6.0 wt.% Al_tot; 0.0001 - 0.05 wt.% Sb; 0.0005 - 0.005 wt.% of £ (Ca + REM); 5 - 100 ppm N; 0 - 2.0 wt.% Si; 0 - 0.01 wt.% S; 0 - 0.1 wt. % P; 0 - 1.0 wt.% Cr; 0 - 2.0 wt.% Ni;0 - 2.0 wt.% Cu; 0 - 0.5 wt.% Mo; 0 - 0.1 wt.% V; 0 - 50 ppm B; 0 - 0.10 wt.% Ti wherein the component has a bimodal grain microstructure consisting of a ferritic matrix phase consisting of delta-ferrite and alpha-ferrite, wherein the delta- ferrite has a grain size of between 5 and 20 gm, wherein the alpha-ferrite has a grainsize at most 3 pm and a second phase consisting of one or more of or bainite, martensite and retained austenite with a grain size of at most 3 pm. But the steel of EP3421629 does not demonstrate the low density steel as well as contains hard faces such as Martensite and Bainite.

The purpose of the invention therefore is to provide a steel sheet presenting a relative density below 7.3, an ultimate tensile strength of at least 600 MPa, and an uniform elongation of at least 17.5%.

In a preferred embodiment, the steel sheet according to the invention presents a relative density equal or below 7.2, a yield strength of at least 450 MPa.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

Carbon content is from 0.12% to 0.25%, more preferably from 0.13% to 0.2% by weight. Carbon is a Gamagenous element which plays a significant role in the formation of residual austenite and also imparts the strength and ductility. The carbon content is advantageously from 0.13% to 0.2% to obtain simultaneously high strength, elongation and stretch flangeability.

Manganese content is present from 3% to 10% by weight. Manganese is an important alloying element in this system, mainly due to the fact that alloying with very high amounts of manganese stabilizes the austenite down to room temperature, which can assist in reaching the target properties such as elongation and yield strength. Manganese, along with Carbon, control the formation of carbides at grain boundaries at high temperature and thereby controls the hot shortness. If the Manganese is present above 10% it may lead to central segregation which is detrimental for the ductility of the steel of present invention. Manganese when present below 3% will not stabilize the residual austenite at room temperature in an adequate amount. Preferred limit for the presence of Manganese is from 4% to 9% and more preferably from 4% to 8%. Aluminum content is present from 3.5% to 6.5% by weight. Aluminum addition to the steel of present invention effectively decreases its density. Aluminum is an alphagenous element and therefore tends to promote the formation of ferrite and in particular of delta ferrite. The aluminum has a relative density of 2.7 and has an influence on the mechanical properties. As the aluminum content increases, the mechanical strength and the elastic limit also increase although the uniform elongation decreases, due to the decrease in the mobility of dislocations. Below 3.5%, the density reduction due to the presence of aluminum becomes less beneficial. Above 6.5%, the presence of ferrite increases beyond the expected limit and affects the present invention negatively. Moreover the presence of Al above 6.5% may forms intermetallics such as Fe-AI, Fea-AI and other (Fe,Mn)AI intermetallics which will impart brittleness to the product that can cause cracking of the steel during cold rolling and may also be detrimental for the toughness of the steel. Preferably, the aluminum content will be limited to strictly less than 6.5% to prevent the formation of brittle intermetallic precipitation, hence the preferred limit is from 4% to 6% and more preferably from 5% to 6%.

Silicon is an optional element that makes it possible to reduce the density of the steel, and effective in solid solution hardening. Nevertheless, its content is limited to 2% by weight because above that level this element has a tendency to form strongly adhesive oxides that generate surface defects. The presence of surface oxides impairs the wettability of the steel and may produces defects during a potential hot-dip galvanizing operation. Therefore, the Si content will preferably be limited below 1 .5%.

Sulfur and phosphorus are impurities that embrittle the grain boundaries. Their respective contents must not exceed 0.03% and 0.1 % by weight so as to maintain sufficient hot ductility.

Nitrogen content must be 0.1 % or less by weight so as to prevent the precipitation of AIN and the formation of volume defects (blisters) during solidification. Niobium may be added as an optional element in an amount of 0.01 % to 0.03% by weight to the steel of present invention to provide grain refinement. The grain refinement allows obtaining a good balance between strength and elongation. But, niobium had a tendency to retard the recrystallization during hot rolling and annealing hence the limit is kept till 0.03%.

Titanium may be added as an optional element in an amount of 0.01 % to 0.2% by weight to the steel of present invention for grain refinement, in a similar manner as niobium.

Copper may be added as an optional element in an amount of 0.01 % to 2.0% by weight to increase the strength of the steel and to improve its corrosion resistance. A minimum of 0.01 % is required to get such effects. However, when its content is above 2.0%, it can degrade the surface aspect.

Nickel may be added as an optional element in an amount of 0.01 to 3.0% by weight to increase the strength of the steel and to improve its toughness. A minimum of 0.01 % is required to get such effects. However, when its content is above 3.0%, nickel causes ductility deterioration.

Molybdenum is an optional element that is present from 0% to 0.5% by weight in the steel of present invention; Molybdenum plays an effective role in improving hardenability and hardness, when added in an amount of at least 0.01 %. Mo is also beneficial for the toughness of the hot rolled product resulting to an easier manufacturing. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.5%. The preferable limit for Molybdenum is from 0% to 0.4% and more preferably from 0 % to 0.3%.

Chromium is an optional element of the steel of present invention, is from 0% to 0.6% by weight. Chromium provides strength and hardening to the steel, but when used above 0.5 % impairs surface finish of the steel. The preferred limit for chromium is from 0.01% to 0.5% and more preferably from 0.01 % to 0.2%.

Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following proportions by weight: Ce ^0.1%, B^0.01 , Ca^0.005, Mg^0.005 and Zr^0.005. Up to the maximum content levels indicated, these elements make it possible to refine the ferrite grain during solidification.

Additionally some trace elements such as Sb, Sn can come from processing of the steel. The maximum limit up to which these elements are acceptable and are not detrimental for the steel of present invention is 0.05% by weight cumulatively or alone, It is preferred by the steel of present invention to have the content of these elements as low as possible and preferably less than 0.03%.

The microstructure of the steel sheet according to the invention comprises, in area fractions, delta ferrite from 60% to 90%, alpha ferrite from 1 % to 10% and residual austenite from 8% to 30% and optionally from 0% to 2% kappa precipitates.

The delta ferrite matrix is present as a primary phase of the steel of the present invention and is present from 60% to 90% by area fraction in the steel of the present invention and preferably from 65% to 90% by area fraction and more preferably from 80% to 90%. Delta ferrite is formed during the solidification of the slab from liquid iron and has generally a coarse grain size. The delta ferrite of the present invention preferably has an average grain size less than 10 pm and more preferably less than 9 pm. The presence of the delta ferrite matrix in the present invention imparts the steel with strength. But the presence of delta ferrite content in present invention above 90% may have negative impacts due to the fact that with the rise in temperature solubility of carbon increases in ferrite. However, carbon in solid solution is highly embrittling for low-density steels because it reduces the mobility of dislocations, which is already low on account of the presence of aluminum. Hence a balance between delta ferrite content and austenite, is very important to impart the present invention with requisite mechanical properties.

Residual Austenite is present in the steel of present invention from 8 to 30% wherein the Residual Austenite of the present invention has an average grain size from 0.6 micron to 2 microns. The preferred average grain size of residual austenite is between 0.6 micron to 1 .2 microns. Residual Austenite is known to have a higher solubility of carbon than ferrite and acts as effective Carbon trap. The Carbon percentage in Austenite is from 0.7% to 1 .5% in weight. Austenite present at a level above 30% produces a negative impact on the present invention by impairing the stretch flangeability. Austenite contributes to the present invention in a very versatile manner depending upon the choice of the temperature of annealing and composition of steel, Austenite of the present invention depicts diverse functionalities such as providing formability and ductility due to TRIP effect. The preferable limit for the Residual Austenite is from 9% to 29% in area fraction.

Alpha-Ferrite of the present invention is present from 1 % to 10% in area fraction. Alpha ferrite is generated by partial transformation of the austenite during cooling after hot rolling and after intercritical annealing and has an average grain size from 0.6 micron to 1 .85 microns. The preferred average grain size of alphaferrite is from 0.6 micron to 1 .2 microns. The alpha ferrite of the present invention imparts the present steel with ductility and elongation. The preferred limit for Alphaferrite is from 2% to 10% in area fraction.

Kappa precipitates in the invention is defined by precipitates whose stoichiometry is (Fe,Mn)3AICx, where x is strictly lower than 1. The area fraction of Kappa precipitates can go up to 2%. Above 2%, the ductility decreases and uniform elongation above 17.5% is not achieved. In addition, uncontrolled precipitation of Kappa around the ferrite grain boundaries may occur, increasing, therefore, the efforts during hot and/or cold rolling. Preferentially, the area fraction of Kappa precipitates should be less than 1 %.

In addition to the above-mentioned microstructure, the microstructure of the low density cold rolled and annealed steel is free from microstructural components, such as Pearlite, Bainite and Martensite.

The 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, which comprises the following steps: The steel sheets according to the present invention are preferably produced through a method in which a semi product, such as slabs, thin slabs, or strip made of a steel according to the present invention having the composition described above, is cast, the cast input stock first to cooled to room temperature and then reheated to a temperature above 1000°C, preferably above 1 150°C and more preferably above1200°C or the casted semi-finished product can be used directly at such a temperature without intermediate cooling. The semi-finished product for the present process is considered as a slab.

The reheated slabs are then undergoing hot rolling. The hot-rolling finishing temperature must be above 750°C and preferably above 770°C.

After the hot-rolling, the strip must be coiled at a temperature below 720°C and preferably from 350°C to 720°C and more preferably the coiling is performed from 700°C to 400°C.

The hot rolled steel strip is cooled to room temperature and then pickling is performed or any other scale removal process is performed.

Then the hot-rolled steel strip is subjected to cold-rolling with a reduction rate between 30% and 90%, preferably between 40% and 90%.

After the cold rolling, the cold rolled steel sheet is annealed by heating the sheet up to an annealing temperature comprised from 840°C to 1000°C and preferably from 850°C to 975°C and more preferably from 850°C to 925°C with a heating rate of at least 1 °C/s and preferably more than 3°C/s , holding it at such annealing temperature during less than 1000 seconds and preferably less than 600 seconds and cooling it at a rate of at least 3°C/s, more preferably of at least 5°C/s and even more preferably of at least 10°C/s. Preferably, this annealing is carried out continuously.

By controlling the annealing temperature and time, a two-phase structure can be obtained during the soaking.

After such annealing step, the steel sheet is cooled to a temperature between room temperature and 480°C and can be optionally held from 100°C to 480°C to be overaged during 1 hour or less and preferably less than 20 minutes and more preferably less than 10 minutes. Thereafter it can be cooled to room temperature.

After annealing, the steel sheet may optionally be submitted to a metallic coating operation to improve its protection against corrosion. The coating process used can be any process adapted to the steel of the invention. Electrolytic or physical vapor deposition can be cited, with a particular emphasis on Jet Vapor Deposition. The metallic coating can be based on zinc or on aluminium, for example.

Preferably, the aluminum-based coating comprises less than 15% Si, less than 5.0% Fe, optionally 0.1 % to 8.0% Mg and optionally 0.1 % to 30.0% Zn, the remainder being Al.

Advantageously, the zinc-based coating comprises 0.01 -8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn.

Examples

The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only, and will display the advantageous features of the present invention.

Steel sheets made of steels with different compositions are gathered in Table 1 wherein the presence of Phosphorus is always less than 100ppm for all the steels, where the steel sheets are produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the steel sheets obtained during the trials and table 4 gathers the result of evaluations of obtained properties.

Table 1 - Compositions

Table one shows underlined values: not according to the invention. Table 2 - Process parameters

The inventive steels and the reference steels are reheated at 1200°C and are air cooled with cooling rate as 9°C/s after hot rolling till the coiling temperature.

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The resulting samples were then analyzed and the corresponding microstructure elements and mechanical properties were respectively gathered in table 3 and 4.

Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as EBSD, XRD or any other microscope for determining microstructural composition of both the inventive steel and reference trials. The area fractions Delta ferrite and Alpha-Ferrite are measured using EBSD. For a given steel sample, an EBSD analysis of at least 4 images corresponding to a magnification of 1000 allows to identify the ferrite grains, their location and size. All grains which grain size is below the cut-off value of 1 .85 pm and are adjacent to austenite grains are counted as alpha ferrite and the corresponding area fraction of such grains is determined. The remaining ferrite grains are counted as delta ferrite and the corresponding area fraction of such grains is determined. The average grain sizes of Delta Ferrite, Residual Austenite and Alpha-Ferrite are also measured by using EBSD. The Residual Austenite area fraction is measured using XRD which are demonstrated in table 3.

Table 3

I = according to the invention; R = reference; underlined values: not according to the invention.

It can be seen from the table above that the trials according to the invention all meet the microstructure targets. Table 4 gathers the mechanical and surface properties of both the inventive steel and reference steel.

Table 4 : mechanical properties of the trials

The yield strength YS, the tensile strength TS and the Uniform elongation UE are measured according to ISO standard ISO 6892-1 , published in October 2009.

To determine the relative density of the steel, the volume of a steel sample is measured by Gas Displacement Pycnometry using helium on one side and its corresponding mass is measured on another side. The mass per volume ratio of the steel in g/cm ³ can then by calculated and further divided by the mass per volume ratio of water at 4°C which amounts to 1 g/cm ³ . The resulting value, which is without a unit, is the relative density of the steel.

I = according to the invention; R = reference; underlined values: not according to the invention.

The examples show that the steel sheets according to the invention are the only one to show all the targeted properties thanks to their specific composition and microstructures.