A super plastic high manganese low density annealed cold rolled steel sheet and a method of production thereof

Abstract

A super plastic high manganese low density annealed cold rolled steel comprising of 0.12% ≤ carbon ≤ 0.5%, 12% ≤ manganese ≤ 20%, 5% ≤ aluminum ≤ 9%, 0% ≤ silicon ≤ 2%, 0% ≤ phosphorus ≤ 0.1%, 0% ≤ sulfur ≤ 0.03%, 0% ≤ nitrogen ≤ 0.1%, 0 ≤ niobium ≤ 0.03%, 0 ≤ titanium ≤ 0.2%, 0% ≤ molybdenum ≤ 0.5%, 0% ≤ chromium ≤ 0.6%, 0% ≤ copper ≤ 2.0%, 0% ≤ nickel ≤ 3.0%, 0% ≤ calcium ≤ 0.005%, 0% ≤ boron ≤ 0.01%, 0% ≤ Magnesium ≤ 0.005%, 0% ≤ Zirconium ≤ 0.005%, 0% ≤ Cerium ≤ 0.1%, and the balance including iron and unavoidable impurities, the steel sheet having a microstructure comprising, in area fraction, 45% to 90% ferrite and 10% to 55% of austenite having an average grain size from 0.5 to 10 microns with an aspect ratio of the residual austenite grains from 1 to 4.5.

Description

[0001] A super plastic high manganese low density annealed cold rolled steel sheet and a method of production thereof

[0002] The present invention deals with a super plastic high manganese low density annealed cold rolled steel sheet and in particular having a duplex microstructure. The steel sheet according to the invention is particularly well suited for the manufacture of part and for use in manufacturing of automobiles such as land motor vehicles.

[0003] 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:

[0004] The first track consists in 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.

[0005] The second track consists in 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 .

[0006] From a viewpoint of improvement in the formability of automotive steel sheets, superplasticity has attracted attention. As used herein, the term “superplasticity” refers to a phenomenon which is caused by grain boundary sliding (other than plastic deformation, dislocation or slip) when materials with fine grain size are tensile-strained at temperatures above half of their melting point so as to exhibit high ductility (50%) at very low strain rate. Namely, at deformation temperatures at which materials exhibit super plasticity, the materials have low strength and very high ductility, and thus it is possible to form or process complex shapes even via a small amount of force. LIS2018 / 0179611 is a superplastic medium manganese steel according to the present invention preferably has a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of aluminum (Al), with the remainder being iron (Fe) and inevitable impurities. In another embodiment, a superplastic medium manganese steel according to the present invention preferably has a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of silicon (Si), with the remainder being iron (Fe) and inevitable impurities. The steel of LIS2018 / 01799611 does not demonstrate the superplastic tensile strength.

[0007] The purpose of the invention therefore is to provide a super plastic high manganese low density annealed cold rolled steel presenting a relative density equal or below 7.3 which simultaneously has:

[0008] - a tensile strength from 30MPa to 200MPa when measured at a temperature of at least 785°C and preferably from 40MPa to 180MPa when measured at a temperature of at least 785°C and even more preferably from 50MPa to 160MPa when measured at a temperature of at least 785°C,

[0009] - a total elongation greater than or equal to 50% when measured at a temperature of at least 785°C with a strain rate equal or less than 0.01 per second and preferably total elongation greater than or equal to 75% at a temperature of at least 785°C with a strain rate equal or less than 0.01 per second.

[0010] In a preferred embodiment, the steel sheet according to the invention presents a yield strength of at least 700 MPa at room temperature.

[0011] In a preferred embodiment, it shows an ultimate tensile strength of 600 MPa or more when measured at room temperature and preferably 880MPa or more.

[0012] In a preferred embodiment, the steel sheet according to the invention presents a yield strength of at least 600 MPa at room temperature, and a total elongation of at least 15% when measured at room temperature. Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

[0013] Carbon content is from 0.12% to 0.5%, more preferably from 0.13% to 0.45% by weight. Carbon is a gamagenous element which plays a significant role in the formation of austenite and also imparts the strength and ductility by strengthening the austenitic grains. Further carbon also contributes in striking a balance between the ferrite-austenite content at high temperatures.

[0014] Manganese content of steel of present invention is from 12 to 20% and preferably from 13 to 19% and more preferably from 14 to 18%. Manganese is an austenite stabilizer that increases strength of steel by enhancing hardenability of the steel and also suppresses austenite-to-martensite transformation during cooling after annealing. Manganese also ensures the fine grains of austenite and ferrite- austenite dual-phase structure during deformation in a high temperature region. At least 12% of manganese should be present in the steel to obtain stable austenite structure in accordance with the present invention. However, twin formation is lowered when the content of the added manganese exceeds 20%, which leads to the increased strength but the decreased ductility at room temperature and also when the content is more than 20% cracks easily occurs during the hot rolling process.

[0015] Aluminum content is present from 5% to 9% by weight. Aluminum addition to the steel of present invention effectively decreases its density. 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 elongation decreases, due to the decrease in the mobility of dislocations. Aluminum is an alphagenous element and therefore tends to promote the formation of ferrite. Aluminum partitions between austenite and ferrite phases at deformation temperatures, and thus contributes to achieving the requisite grain size. Aluminum also increases the ferrite-austenite dual phase temperature region to enable a ferrite-austenite dual-phase to be formed during deformation at superplastic temperatures. Below 5%, the density reduction due to the presence of aluminum becomes less beneficial. Above 9%, the presence of ferrite increases beyond the expected limit and affects the present invention negatively. Moreover the presence of Al above 9% may forms intermetallics such as Fe-AI, Fes-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 9% to prevent the formation of brittle intermetallic precipitation, hence the preferred limit is from 5.5% to 8% and more preferably from 6% to 7.5%.

[0016] 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%.

[0017] 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.

[0018] 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.

[0019] Niobium may be added as an optional element in an amount of up to 0.03% and preferably from 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 has a tendency to retard the recrystallization during hot rolling and annealing hence the limit is kept till 0.03%.

[0020] Titanium may be added as an optional element in an amount of up to 0.2% and preferably from 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 up to 2.0% and preferably from 0.01 % to 2.0% by weight to increase the strength of the steel and to improve its corrosion resistance. However, when its content is above 2.0%, it can degrade the surface aspect.

[0021] Nickel may be added as an optional element in an amount of up to 3.0% and preferably from 0.01 to 3.0% by weight to increase the strength of the steel and to improve its toughness. However, when its content is above 3.0%, nickel causes ductility deterioration.

[0022] Molybdenum is an optional element that can be present up 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%.

[0023] Chromium is an optional element of the steel of present invention, that can be present up 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%.

[0024] 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^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.

[0025] 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 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 Copper, Nickel, Molybdenum, Zinc, Antimony, Arsenic and Lead, up to a cumulated amount of 1 %, in addition to the amounts obtained by a blast furnace route.

[0026] The microstructure of the steel sheet according to the invention comprises, in area fractions, ferrite from 45% to 90%, and residual austenite from 10% to 55%.

[0027] The ferrite matrix is present as a primary phase of the steel of the present invention and is present from 45% to 90% by area fraction in the steel of the present invention and preferably from 50% to 88% by area fraction and more preferably from 54% to 87%. The ferrite of the present invention preferably has an average grain size less than 30 pm and more preferably less than 25 pm. The ferrite of the present invention preferably has an aspect ratio from 1 to 4.5 and preferably from 1.2 to 4 and more preferably from 1.4 to 3.5. The presence of the ferrite matrix in the present invention imparts the steel with strength at high temperature. But the presence of 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 ferrite content and austenite, is very important to impart the present invention with requisite superplasticity.

[0028] Austenite is present in the steel of present invention from 10 to 55% wherein the Austenite of the present invention has an average grain size from 0.5 micron to 10 microns. The preferred average grain size of residual austenite is from 1 micron to 8 microns. Residual Austenite of the present invention has an aspect ratio from 1.5 to 4.5 and preferably from 1.6 to 4 and more preferably from 1.6 to 3.5. The grain size and aspect ratio in accordance of the present invention is achieved even after deformation in the high temperature region is due to the different compositional partitioning of Manganese and Aluminum in the ferrite and in the austenite respectively. This partitioning slows down the grain coarsening in duplex alloys because it necessitates substantial diffusional mass transport for the isolated grains of each phase to coarsen. 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 preferably from 0.7% to 1.5% in weight. 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 and yield strength. The preferable limit for the Residual Austenite is from 12% to 50% in area fraction and more preferably from 13% to 48% in area fraction.

[0029] In addition to the above-mentioned microstructure, the microstructure of a super plastic high manganese low density annealed cold rolled steel sheet is free from microstructural components, such as Pearlite, Bainite and Martensite.

[0030] 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:

[0031] 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 1150°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.

[0032] 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 150°C to 720°C and more preferably the coiling is performed from 200°C to 600°C.

[0033] The hot rolled steel strip is cooled to room temperature and then pickling is optionally performed or any other optional scale removal process is performed.

[0034] Then the hot-rolled steel strip is subjected to cold-rolling with a reduction rate from 30% to 90%, preferably from 40% to 90%.

[0035] After the cold rolling, the cold rolled steel sheet is annealed by heating the sheet up to an annealing temperature comprised from 680°C to 950°C and preferably from 700°C to 900°C and more preferably from 750°C to 890°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.

[0036] By controlling the annealing temperature and time, a two-phase structure can be obtained during the soaking.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] Advantageously, the zinc-based coating comprises 0.01 -8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn. Examples

[0041] 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.

[0042] Steel sheets made of steels with different compositions are gathered in Table 1 , where the steel sheets are produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the mechanical properties specifically the superplastic properties of the steel sheets obtained during the trials and table 4 gathers the result of microstructure of the inventive steel after they have demonstrated superplastic properties.

[0043] Table 1 - Compositions

[0044] Table 2 - Process parameters The steels are reheated at 1180°C and are air cooled with cooling rate of 9°C / s after hot rolling till the coiling temperature. Both the examples are cooled to room temperature after annealing. The resulting samples were then analyzed, and the corresponding microstructure elements and mechanical properties were respectively gathered in table 3 and 4.

[0045] Table 3 gathers the mechanical and surface properties of the inventive steel. Table 3 shows the properties of the inventive steel measured at two different temperature that is at 785°C and 880°C at different strain rates to demonstrate the super plasticity of steel of present invention.

[0046] Table 3 : mechanical properties of the trials

[0047] The yield strength YS, the tensile strength TS and the total elongation TE are measured according to ISO standard ISO 6892-1 , published in October 2009. The relative density of the steels is measured by Gas Displacement Pycnometry System and the gas used is Helium Gas.

[0048] Table 4 gathers the results of test conducted in accordance with standards on different microscopes such as SEM, EBSD, XRD or any other microscope for determining microstructural composition of the inventive steels. The area fractions of Ferrite are measured using SEM. The average grain sizes of Residual Austenite and Ferrite are also measured by using SEM. The Austenite area fraction is measured using XRD. Table 4 shows the microstructure of the inventive steels measured on the steels after the inventive steels have demonstrated the super plastic behavior. Aspect ratio is the ratio of the longest intercept grain dimension maximum Feret diameter(Fmax) to the longest intercept grain dimension measured at 90 ° of said Fmax ( Fmax90°).

[0049] Aspect ratio = (Fmax) / ( Fmax90°) Table 4

[0050] It can be seen from the table above that the trials according to the invention all meet the microstructure targets.

Claims

CLAIMS1 . A super plastic high manganese low density annealed cold rolled steel comprising by weight:0.12% < carbon < 0.5%, 12% < manganese < 20%, 5% < aluminum < 9%, 0% < silicon < 2%, 0% < phosphorus < 0.1 %, 0% < sulfur < 0.03%, 0% < nitrogen < 0.1 %, and optionally one or more of the following elements 0 < niobium < 0.03%, 0 < titanium < 0.2%,0% < molybdenum < 0.5%, 0% < chromium < 0.6%, 0% < copper < 2.0%, 0% < nickel < 3.0%, 0% < calcium < 0.005%, 0% < boron < 0.01 %,0% < Magnesium < 0.005%, 0% < Zirconium < 0.005%,0% < Cerium < 0.1 %, and the balance including iron and unavoidable impurities, the steel sheet having a microstructure comprising, in area fraction, 45% to 90% ferrite and 10% to 55% of austenite having an average grain size from 0.5 to 10 microns with an aspect ratio of the residual austenite grains from 1 to 4.5.

2. A steel sheet according to claim 1 , wherein the carbon content is comprised from 0.13% to 0.45%.

3. A steel sheet according to claims 1 or 2, wherein the manganese content is comprised from 13% to 19%.

4. A steel sheet according to claims 1 to 3, wherein the aluminum content is comprised from 5.5% to 8%.

5. A steel sheet according to anyone of claims 1 to 4, wherein the ferrite content is from 50% to 88%.

6. A steel sheet according to anyone of claims 1 to 5, wherein the austenite content is from 12% to 50%.

7. A steel sheet according to anyone of claims 1 to 6, wherein the tensile strength is from 30MPa to 200MPa when measured at a temperature of at least 785°C.

8. A method for producing a steel sheet comprising the following steps:- feeding a slab which composition is according to claims 1 to 4,- reheating such slab at a temperature above 1000°C and hot rolling it with a final rolling temperature of at least 750°C,- coiling the hot rolled steel sheet at a temperature below 720°C,- cooling the said hot rolled sheet;- optionally performing pickling on said hot rolled steel sheet;- cold rolling the said hot rolled steel sheet with a reduction rate from 30 to 90% to obtain a cold rolled steel sheet;- annealing said cold rolled steel sheet by heating the steel sheet from room temperature to an annealing temperature from 680°C to 950°C, with a heating rate of at least 1 °C / s,- then performing annealing during less than 1000 seconds, then cooling the cold rolled steel sheet to a cooling stop temperature from 480°C to room temperature with a cooling rate of at least 3°C / s, and optionally holding the cold rolled steel sheet from 100°C to 480°C during 1 hour or less,- thereafter cooling the cold rolled steel sheet to room temperature to obtain a super plastic high manganese low density steel.

9. A method according to claim 8, wherein the annealing temperature is from 700°C to 900°C.

10. A method according to anyone of claims 9 and 10, wherein the coiling temperature is from 150°C to 720°C.

11. A method according to anyone of claims 8 to 10, wherein the holding time of the annealing is less than 600 seconds.

12. A method according to anyone of claims 8 to 11 , wherein the heating rate for the annealing is more than 3°C / s.

13. A method according to anyone of claims 8 to 12, wherein the hot-rolling finishing temperature is above 770°C.

14. A method according to anyone of claims 8 to 13, comprising further a final coating step.

15. Use of a steel sheet according to anyone of claims 1 to 7 or obtainable according to the method of anyone of claims 8 to 14 for manufacturing a structural or safety part of a vehicle.

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