Cold rolled superplastic high manganese low density steel sheet without heat treatment and method for producing the same

Abstract

An un-heat treated cold rolled superplastic high manganese low density steel sheet comprising: 0.12% ≤ carbon ≤ 0.5%, 12% ≤ manganese ≤ 20%, 5% ≤ aluminum ≤ 9%, 0% ≤ silicon ≤ 2%, 0% ≤ phosphorous ≤ 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%, the balance comprising iron and unavoidable impurities, said steel sheet having a microstructure comprising 45% to 90% ferrite and 10% to 55% austenite in area fraction, said austenite having an average grain size of 0.5 microns to 25 microns, and a residual austenite grain having an aspect ratio of 1 to 4.5.

Description

Technical Field

[0001] This invention relates to cold-rolled, superplastic, high-manganese, low-density steel sheets without heat treatment, particularly those with a dual-phase microstructure. The steel sheets according to the invention are especially suitable for manufacturing components and for use in the manufacture of automobiles, such as land vehicles. Background Technology

[0002] Environmental constraints are forcing automakers to continuously reduce CO2 emissions from their vehicles. To this end, automakers have several options, the primary ones being either reducing vehicle weight or improving the efficiency of their engine systems. Progress is typically made by combining these two approaches. This invention relates to the first option, namely, reducing the weight of motor vehicles. In this very specific area, there are dual-track alternatives:

[0003] The first approach involves increasing the mechanical strength of steel while reducing its thickness. Unfortunately, this solution has its limitations, as the rigidity of some automotive components decreases excessively, and acoustic problems arise that cause discomfort to passengers, not to mention the inevitable loss of ductility with increased mechanical strength.

[0004] The second approach involves reducing the density of steel by alloying it with other lighter metals. Among these alloys, low-density alloys possess attractive mechanical and physical properties while enabling significant weight reduction.

[0005] From the perspective of improving the formability of automotive steel sheets, superplasticity has attracted attention. As used in this article, the term "superplasticity" refers to the phenomenon that occurs when a material with a fine grain size is subjected to tensile strain at a temperature above half its melting point, resulting in high ductility (50%) at extremely low strain rates due to grain boundary slip (rather than plastic deformation, dislocation, or slip). That is, at the deformation temperature at which a material exhibits superplasticity, it possesses low strength and extremely high ductility, thus allowing it to be formed or processed into complex shapes even with very small forces.

[0006] US2018 / 0179611 is a superplastic medium-manganese steel, which, according to the invention, preferably has a composition comprising 4 to 8 wt% manganese (Mn) and 3 wt% or less (excluding 0 wt%) aluminum (Al), with the remainder being iron (Fe) and unavoidable impurities. In another embodiment, the superplastic medium-manganese steel according to the invention preferably has a composition comprising 4 to 8 wt% manganese (Mn) and 3 wt% or less (excluding 0 wt%) silicon (Si), with the remainder being iron (Fe) and unavoidable impurities. The steel of US2018 / 01799611 does not exhibit superplastic tensile strength. Summary of the Invention

[0007] Therefore, the object of the present invention is to provide a cold-rolled, superplastic, high-manganese, low-density steel sheet without heat treatment, which exhibits a relative density equal to or less than 7.3, and simultaneously possesses:

[0008] - Tensile strength of 20 MPa to 200 MPa when measured at a temperature of at least 785°C, and preferably 25 MPa to 180 MPa when measured at a temperature of at least 785°C, even more preferably 25 MPa to 160 MPa when measured at a temperature of at least 785°C.

[0009] - A total elongation greater than or equal to 75% when measured at a strain rate of 0.01 per second at a temperature of at least 785°C, and preferably greater than or equal to 100% when measured at a strain rate of 0.01 per second at a temperature of at least 785°C.

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

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

[0012] In a preferred embodiment, the steel plate according to the invention exhibits a yield strength of at least 600 MPa at room temperature and a total elongation of at least 15% when measured at room temperature.

[0013] Other features and advantages of the invention will become apparent from the following detailed description of the invention. Detailed Implementation

[0014] The carbon content is 0.12% to 0.5% by weight, more preferably 0.13% to 0.45%. Carbon is an austenite-forming element, playing an important role in austenite formation and also imparting strength and ductility by strengthening austenite grains. In addition, carbon helps to achieve a balance between ferrite and austenite content at high temperatures.

[0015] The steel of this invention has a manganese content of 12% to 20%, preferably 13% to 19%, and more preferably 14% to 18%. Manganese is an austenite stabilizer that increases the strength of the steel by enhancing its hardenability and also inhibits the austenite-to-martensite phase transformation during cooling after hot rolling. Manganese also ensures fine grains of the austenite and ferrite-austenite dual-phase structure during deformation in high-temperature regions. According to the invention, at least 12% manganese should be present in the steel to obtain a stable austenitic structure. However, when the added manganese content exceeds 20%, twinning formation decreases, resulting in increased strength but decreased ductility at room temperature, and when the content exceeds 20%, cracking is more likely to occur during the hot rolling process.

[0016] The aluminum content is present in the form of 5% to 9% by weight. Adding aluminum to the steel of this invention effectively reduces its density. Aluminum has a relative density of 2.7 and influences mechanical properties. With increasing aluminum content, mechanical strength and elastic limit increase due to decreased dislocation mobility, while elongation decreases. Aluminum is a ferrite-forming element and therefore tends to promote ferrite formation. Aluminum distributes between the austenite and ferrite phases at deformation temperatures, thereby contributing to achieving the desired grain size. Aluminum also expands the ferrite-austenite dual-phase temperature range, enabling the formation of the ferrite-austenite dual phase during deformation at superplastic temperatures. Below 5%, the density reduction due to the presence of aluminum becomes less advantageous. Above 9%, the presence of ferrite increases beyond the expected limit and negatively impacts the invention. Furthermore, the presence of more than 9% Al may form intermetallic compounds, such as Fe-Al, Fe3-Al, and other (Fe,Mn)Al intermetallic compounds, which will impart brittleness to the product, potentially causing the steel to crack during cold rolling, and may also be detrimental to the steel's toughness. Preferably, the aluminum content will be strictly limited to less than 9% to prevent the formation of brittle intermetallic compound deposits, thus the preferred limit is 5.5% to 8%, and more preferably 6% to 7.5%.

[0017] Silicon is an optional element that makes it possible to reduce the density of steel and is effective in solid solution strengthening. However, its content is limited to 2% by weight because above this level, the element tends to form strongly adhering oxides that produce surface defects. The presence of surface oxides impairs the wettability of the steel and can potentially cause defects during hot-dip galvanizing operations. Therefore, it is preferable to limit the Si content to below 1.5%.

[0018] Sulfur and phosphorus are impurities that embrittle grain boundaries. Their respective contents must not exceed 0.03% by weight and 0.1% by weight to maintain sufficient thermal ductility.

[0019] The nitrogen content must be 0.1% by weight or less to prevent AlN from precipitating and forming volume defects (bubbles) during solidification.

[0020] Niobium may be added as an optional element to the steel of the present invention in an amount of up to 0.03 wt%, and preferably from 0.01 wt% to 0.03 wt%, to provide grain refinement. Grain refinement allows for a good balance between strength and elongation. However, niobium has a tendency to delay recrystallization during hot rolling, so this limit is kept below 0.03%.

[0021] Titanium may be added as an optional element to the steel of the present invention in an amount of up to 0.2% by weight, and preferably from 0.01% to 0.2% by weight, for grain refinement in a manner similar to that of niobium.

[0022] Copper can be added as an optional element in amounts up to 2.0% by weight, and preferably from 0.01% to 2.0% by weight, to increase the strength of steel and improve its corrosion resistance. However, when its content exceeds 2.0%, it will degrade the surface appearance.

[0023] Nickel can be added as an optional element in amounts up to 3.0% by weight, and preferably from 0.01% to 3.0% by weight, to improve the strength and toughness of steel. However, when its content exceeds 3.0%, nickel leads to a deterioration in ductility.

[0024] Molybdenum is an optional element that can be present in the steel of the present invention at a maximum of 0.5% by weight; when added in an amount of at least 0.01%, molybdenum plays an effective role in improving hardenability and hardness. Mo also benefits the toughness of hot-rolled products, thereby making manufacturing easier. However, excessive addition of molybdenum increases the cost of alloying element addition, and therefore, for economic reasons, its content is limited to 0.5%. The preferred limit for molybdenum is 0% to 0.4%, and more preferably 0% to 0.3%.

[0025] Chromium is an optional element in the steel of the present invention, and it can be present at a maximum of 0.6% by weight. Chromium provides strength and hardening to the steel, but when used at a level higher than 0.5%, it impairs the surface finish of the steel. The preferred limit for chromium is 0.01% to 0.5%, and more preferably 0.01% to 0.2%.

[0026] Other elements such as cerium, boron, magnesium, or zirconium may be added, alone 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 shown, these elements enable the refinement of ferrite grains during solidification.

[0027] In addition, some trace elements such as Sb and Sn may originate from the steel processing. The maximum acceptable level of these elements that will not be detrimental to the steel of the present invention is 0.05% by weight, either cumulatively or individually. The steel of the present invention preferably contains these elements as low as possible, and preferably less than 0.03%.

[0028] The remaining components of steel are iron and unavoidable impurities produced during the smelting process, depending on the process route. In production routes using blast furnaces, the level of unavoidable impurities is very low. In production routes using electric arc furnaces loaded with scrap steel, in addition to the amounts obtained through the blast furnace route, the steel plates may contain residual elements from such scrap steel, such as copper, nickel, molybdenum, zinc, antimony, arsenic, and lead, with cumulative amounts up to 1%.

[0029] The microstructure of the steel plate according to the invention comprises 45% to 90% ferrite and 10% to 55% retained austenite in area fraction.

[0030] The ferrite matrix exists as the main phase of the steel of the present invention, and is present in the steel of the present invention at a concentration of 45% to 90% by area, preferably 48% to 88% by area, and more preferably 50% to 84% by area. The average grain size of the ferrite of the present invention is preferably less than 30 µm, and more preferably less than 25 µm. The ferrite of the present invention preferably has an aspect ratio of 1 to 4.5, preferably 1.2 to 4, and more preferably 1.4 to 3.5. The presence of the ferrite matrix in the present invention imparts strength to the steel at high temperatures. However, the presence of a ferrite content higher than 90% in the present invention may have a negative impact due to the fact that the solubility of carbon in ferrite increases with increasing temperature. However, carbon in the solid solution has a highly embrittlement effect on low-density steel because it reduces the dislocation mobility, which is already low due to the presence of aluminum. Therefore, the balance between the ferrite content and austenite is very important for imparting the superplasticity required by the present invention.

[0031] Austenite is present in the steel of the present invention at a concentration of 10% to 55%, wherein the austenite of the present invention has an average grain size of 0.5 micrometers to 25 micrometers. The preferred average grain size of the retained austenite is 2 micrometers to 23 micrometers. The retained austenite of the present invention has an aspect ratio of 1.5 to 4.5, preferably 1.6 to 4, more preferably 1.6 to 3.5. The grain size and aspect ratio according to the present invention can be achieved even after deformation in the high-temperature region due to the different compositional distribution of manganese and aluminum in ferrite and austenite, respectively. This distribution slows down grain coarsening in the dual-phase alloy, as it requires a large amount of diffusion mass transport to coarsen the isolated grains of each phase. Retained austenite is known to have a higher carbon solubility than ferrite and acts as an effective carbon trap. The carbon percentage in the austenite is preferably 0.7 wt% to 1.5 wt%. The austenite of the present invention exhibits a variety of functions, such as providing formability as well as ductility and yield strength. The preferred limit for retained austenite is 12% to 52% in area fraction, and more preferably 15% to 50% in area fraction.

[0032] In addition to the microstructures mentioned above, the microstructure of untreated cold-rolled superplastic high-manganese low-density steel sheets does not contain microstructure components such as pearlite, bainite, and martensite.

[0033] The steel plate according to the invention can be produced by any suitable manufacturing method, and one can be defined by those skilled in the art. However, it is preferred to use the method according to the invention, which includes the following steps:

[0034] The steel plate according to the invention is preferably produced by a method in which a semi-finished product, such as a slab, sheet, or strip, made of the steel according to the invention having the above-described composition is cast. The input raw material for casting is first cooled to room temperature and then heated to a temperature of 1000°C or higher, preferably 1150°C or higher, and more preferably 1200°C or higher. Alternatively, the semi-finished product can be used directly at such a temperature without intermediate cooling. The semi-finished product used in this process is considered a slab.

[0035] The reheated slab is then subjected to hot rolling. The finishing temperature of the hot rolling must be above 750°C, and preferably above 770°C.

[0036] After hot rolling, the strip must be coiled at a temperature below 720°C, preferably at 150°C to 720°C, and more preferably at 200°C to 600°C.

[0037] The hot-rolled steel strip is cooled to room temperature and then optionally pickled or subjected to any other optional oxide scale removal process.

[0038] The hot-rolled steel strip is then cold-rolled at a reduction rate of 30% to 90%, preferably 40% to 90%.

[0039] After such cold rolling, the steel sheet can optionally be aged at 100°C to 480°C for 1 hour or less, preferably less than 20 minutes, more preferably less than 10 minutes. Afterward, it can be cooled to room temperature.

[0040] After cold rolling, the steel sheet can optionally undergo a metal coating operation to improve its corrosion protection. The coating process used can be any process suitable for the steel of this invention. Examples include electrolytic or physical vapor deposition, with particular emphasis on spray vapor deposition. For example, the metal coating can be based on zinc or aluminum.

[0041] 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, with the remainder being Al.

[0042] Advantageously, the zinc-based coating contains 0.01-8.0% Al, optionally 0.2-8.0% Mg, with the remainder being Zn.

[0043] Example

[0044] The tests, examples, illustrative illustrations, and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, and will show advantageous features of the invention.

[0045] Steel plates made from steels with different compositions are summarized in Table 1, wherein the steel plates were produced according to the process parameters specified in Table 2. Subsequently, Table 3 summarizes the mechanical properties, particularly the superplastic properties, of the steel plates obtained during the experiment, and Table 4 summarizes the microstructure results of the invented steels after they exhibited superplastic properties.

[0046] Table 1 – Composition

[0047]

[0048] Table 2 – Process Parameters

[0049] The steel is reheated at 1180°C and then air-cooled to the coiling temperature at a cooling rate of 9°C / second after hot rolling.

[0050]

[0051] The obtained samples were then analyzed, and the corresponding microstructure elements and mechanical properties are summarized in Tables 3 and 4, respectively.

[0052] Table 3 summarizes the mechanical and surface properties of the invented steel. Table 3 shows the properties of the invented steel measured at different strain rates at two different temperatures (i.e., 785°C and 880°C) to demonstrate the superplasticity of the invented steel.

[0053] Table 3: Mechanical properties of the test

[0054] Yield strength YS, tensile strength TS, and total elongation TE were measured according to ISO standard ISO 6892-1, published in October 2009. The relative density of the steel was measured using a gas-displacement hydrometer system with helium as the gas.

[0055]

[0056] Table 4 summarizes the test results used to determine the microstructure composition of the invented steel, performed according to standards on various microscopes such as SEM, EBSD, XRD, or any other microscope. The ferrite area fraction was measured using SEM. The average grain size of austenite and ferrite was also measured using SEM. The austenite area fraction was measured using XRD. Table 4 shows the microstructure of the invented steel as measured after it exhibited superplastic properties. Aspect ratio is the ratio of the longest intercept grain size (maximum Feret diameter, Fmax) to the longest intercept grain size (Fmax90°) measured at 90° of said Fmax. Aspect ratio = (Fmax) / (Fmax90°)

[0057] Table 4

[0058]

[0059] As can be seen from the table above, all the experiments according to the present invention meet the microstructure target.

Claims

1. A cold-rolled, superplastic, high-manganese, low-density steel sheet without heat treatment, 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%, The balance includes iron and unavoidable impurities. The steel plate has a microstructure comprising 45% to 90% ferrite and 10% to 55% austenite by area fraction, the austenite having an average grain size of 0.5 micrometers to 25 micrometers and an aspect ratio of 1 to 4.5 for the retained austenite grains.

2. The steel plate according to claim 1, wherein the carbon content is from 0.13% to 0.45%.

3. The steel plate according to claim 1 or 2, wherein the manganese content is 13% to 19%.

4. The steel plate according to claims 1 to 3, wherein the aluminum content is 5.5% to 8%.

5. The steel plate according to any one of claims 1 to 4, wherein the ferrite content is 48% to 88%.

6. The steel plate according to any one of claims 1 to 5, wherein the austenite content is 12% to 52%.

7. The steel plate according to any one of claims 1 to 6, wherein the tensile strength is 20 MPa to 200 MPa when measured at a temperature of at least 785°C.

8. A method for producing steel plates, comprising the following steps: - Provides a slab comprising the form described in claims 1 to 4; - The slab is reheated at a temperature above 1000°C and hot-rolled at a final rolling temperature of at least 750°C. - Hot-rolled steel sheets are coiled at temperatures below 720°C; - Cool the hot-rolled steel sheet; - Optionally, the hot-rolled steel sheet may be pickled; - The hot-rolled steel sheet is cold-rolled at a reduction rate of 30% to 90% to obtain a cold-rolled steel sheet; - Then, the cold-rolled steel sheet can optionally be held at a temperature of 100°C to 480°C for 1 hour or less; - The cold-rolled steel sheet is then cooled to room temperature to obtain a cold-rolled superplastic high-manganese low-density steel sheet without heat treatment.

9. The method of claim 8, wherein the final hot rolling temperature is at least 770°C.

10. The method according to any one of claims 9 and 10, wherein the winding temperature is from 150°C to 720°C.

11. The method according to any one of claims 8 to 10, further comprising a final coating step.

12. Use of the steel sheet according to any one of claims 1 to 7, or the steel sheet obtainable by the method according to any one of claims 8 to 11, in the manufacture of structural or safety components of a vehicle.

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