The present invention deals with a low density steel. The steel according to the invention is particularly well suited for the manufacture of parts for vehicles such as land motor vehicles or for further processing to manufacture a cold rolled steel sheet and parts of the vehicles thereafter.

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 such as Aluminum. Among these kind of steels low- density steels have attractive mechanical and physical properties while making it possible to significantly reduce the weight for a hot rolled steel. In particular, US2003/014591 1 which discloses a Fe-AI-Mn-Si light steel having good formability and high strength. However, the steel of US 2003/014591 1 does not demonstrate hardness of the low density steel which is an important property for processing the steel hence the steel of US 2003/014591 1 does not allow taking full advantage of their low density for parts of all kinds of geometry.

Hence, the purpose of the invention therefore is to provide a steel sheet presenting a relative density below 7.3, a hardness of the steel less than 280 Hv and microstructure grains having less than 4GPa nano-hardness must be more than 45% and microstructure grains having nano-hardness of more than 5GPa must be less than 22%.

In the preferred embodiment of the invention therefore is to provide a steel sheet presenting a relative density equal to or below 7.2, a hardness of the steel less than 275Hv and microstructure grains having less than 4GPa nano-hardness must be more than 50% and microstructure grains having nano-hardness of more than 5GPa must be less than 20%.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 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 can be added as 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 can be added as 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 calcium, cerium, boron, magnesium or zirconium can be added individually or in combination in the following proportions by weight: Ce ^0.1 %, B^O.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, ferrite from 55% to 80%, 15% to 50% austenite and martensite from 0% to 10%.

The ferrite matrix is present as a primary phase of the steel of the present invention and is present from 55% to 80% by area fraction in the steel of the present invention and preferably from 58% to 80% by area fraction and more preferably from 58% to 78%. Ferrite is formed during the solidification of the slab from liquid iron and cooling after hot rolling and the ferrite of the present invention preferably forms as a banded structure having an average band thickness from 1 micron to 70 microns and more preferably from 1 microns to 65 microns and it is more preferably from 2 microns to 62 microns. The presence of the ferrite matrix in the present invention imparts the steel with strength. But the presence of ferrite content in present invention above 80% 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 ferrite content and austenite, is very important to impart the present invention with requisite mechanical properties.

Austenite is an essential microconstituent for the steel of present invention and is present from 15 to 50% wherein the Austenite of the present invention preferably has an average grain size from 2 microns to 25 microns and more preferably from 2 microns to 22 microns. Austenite is known to have a higher solubility of carbon than ferrite and acts as effective Carbon trap. Austenite present at a level above 50% produces a negative impact on the present invention by impairing the formation of ferrite thereby impairing the sought properties of the steel of present invention. Hence the preferable limit for the presence of austenite is from 15% to 45% and more preferably from 18% to 45%

Martensite is an optional constitutes of the steel of present invention and is present from 0% to 10% of the microstructure by area fraction. Martensite is formed during the cooling after coiling and particularly below Ms temperature The martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of martensite is between 0% and 8% and more preferably between 0% and 6%.

In addition to the above-mentioned microstructure, the microstructure of the low density hot rolled steel is free from microstructural components, such as Bainite and Kappa Precipitates.

45% or more grains of microstructure of the present steel have nanohardness of less than 4G Pa, this feature of the steel of present invention facilitates further processing of the hot rolled steel for diverse manufacturing process such as cold rolling, extrusion, pressing, stamping, hydroforming, hemming, punching and drawing. By keeping the nano hardness of 45% or more grains of microstructure below 4GPa, the steel of present invention ensures the hot rolled steel is not very hard for the manufacturing process and simultaneously the steel must not be too soft for the mechanical properties. The preferable limit for the grains to have nanohardness of less than 4G Pa is more than 50% and more preferably 55%.

In addition to above the hot rolled steel of the present invention also limits the presence of the grains of microstructure of the present steel have nano-hardness of more than 5GPa to 22% or less. This is done because whenever the grains having nano-harness of 5GPa are present more than 22%, it imparts too much of wear on the industrial machinery used in manufacturing of the parts for automotive industry such as it is not possible to perform cold rolling without any prior thermal treatment when the steel contains 22% or more grains of microstructure having nanohardness 5GPa or more. Hence it is preferred that grains having nano-hardness 5GPa or more less than 20% and more preferably less than 18%. In addition to the above-mentioned microstructure, the microstructure of the hot rolled low density steel is free from microconstituent components such as Kappa precipitates and bainite.

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 at least 750°C and preferably at least 770°C. The hot rolling finishing is kept above 750°C to ensure that hot rolling must be completed in a region having delta ferrite from 50% to 100%.

The hot rolled strip obtained in this manner is then cooled, such cooling starting immediately after the finishing of hot rolling and the hot rolled strip being cooled from finishing of hot rolling to a coiling temperature range from 380°C to 480°C at a cooling rate CR1 from 10°C/s to 150°C/s. In a preferred embodiment, the cooling rate CR1 is from 20°C/s to 120°C/s and more preferably the cooling rate CR1 is between 30°C/s and 100°C/s. During this cooling the steel of present invention starts transformation from Delta ferrite into Ferrite and Austenite.

Thereafter, the hot rolled steel must be coiled at a temperature from 380°C to 480°C and preferably the coiling is performed from 390°C to 470°C and more preferably the coiling is performed from 400°C to 460°C .

The hot rolled steel is cooled from coiling temperature to room temperature at a cooling rate CR2 from 1 °C/h to 50°C/h. In a preferred embodiment, the cooling rate CR2 is from 10°C/h to 40°C/h.to obtain a low density hot rolled steel. Thereafter an optional pickling or any other scale removal process may be performed to facilitate further processing for the hot rolled steel to be manufacturing into the part of the vehicle such as cold rolling, cutting into sheet pile, forming, etc.

After cooling to room temperature, the hot rolled steel 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.

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 XRD and different microscopes such as Optical Microscope, Scanning Electron Microscope for determining microstructural composition of both the inventive steel and reference trials. The fraction of austenite was measured by XRD. The area fractions of kappa carbides and martensite were determined by Optical and Scanning Electron Microscopy through the analysis of at least four images. The area fraction of Ferrite was calculated by subtracting to 100% the sum of the fractions of all the other phases.

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 hardness is measured by Vickers hardness test conducted as per the ISO6507 standards. 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. To determine the nano-indentation, the samples are prepared by OPU polishing (8min, dilution to 50%) before performing nanoindentation test, at a load of 3mN. A spacing of 8 pm is chosen between the points of the indentation grid. 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.