KR20230100739A - Low-density cold-rolled and annealed steel sheet, method for producing same, and use of such steel for producing vehicle parts - Google Patents

Abstract

The present invention relates to a low-density cold-rolled and annealed steel sheet, which contains, by weight, 0.12% ≤ carbon ≤ 0.25%, 3% ≤ manganese ≤ 10%, 3.5% ≤ aluminum ≤ 6.5%, 0% ≤ phosphorus ≤ 0.1% , 0% ≤ sulfur ≤ 0.03%, 0% ≤ nitrogen ≤ 0.1%, 0% ≤ silicon ≤ 2%, 0.01% ≤ niobium ≤ 0.03%, 0.01% ≤ titanium ≤ 0.2%, 0% ≤ molybdenum ≤ 0.5%, 0 % ≤ Chromium ≤ 0.6%, 0.01% ≤ Copper ≤ 2.0%, 0.01% ≤ Nickel ≤ 3.0%, 0% ≤ Calcium ≤ 0.005%, 0% ≤ Boron ≤ 0.01%, 0% ≤ Magnesium ≤ 0.005%, 0% ≤ zirconium ≤ 0.005%, 0% ≤ cerium ≤ 0.1%, the remainder comprising iron, the steel sheet comprising, in area fraction, 60 to 90% delta ferrite, 8 with an average grain size of 0.6 to 2 microns % to 30% retained austenite, 1.0% to 10% alpha-ferrite with an average grain size of 0.6 to 2 microns and 0% to 2% kappa precipitates (Fe,Mn)3AICx with x strictly lower than 1 It has a microstructure that contains

Description

Low-density cold-rolled and annealed steel sheet, method for producing same, and use of such steel for producing vehicle parts

The present invention relates to low-density steel sheets, particularly to duplex microstructures. The steel sheet according to the present invention is particularly suitable for the manufacture of interior or exterior panels for vehicles such as land vehicles.

Environmental regulations are forcing automakers to continuously reduce the CO2 emissions of their vehicles. To this end, automakers have several options, the main ones being to reduce vehicle weight or improve the efficiency of engine systems. Progress is often achieved by a combination of the two approaches. The present invention relates to a first option, namely reducing the weight of the vehicle. In this very specific field, there are two track alternatives:

The first track is to reduce the thickness of the steel while increasing the mechanical strength level of the steel. Unfortunately, this solution is limited due to the absurd reduction in the rigidity of certain automotive parts and the occurrence of acoustic problems causing discomfort to passengers, not to mention the unavoidable loss of ductility associated with the increase in mechanical strength.

The second track is to lower the density of the steel by alloying it with other, lighter metals. Among these alloys, low-density alloys can greatly reduce weight while having attractive mechanical and physical properties.

In particular, EP3421629 is a patent claiming a high-strength cold-rolled and heat-treated steel strip, sheet, blank or hot-formed product having a bimodal microstructure, comprising forming and casting a melt into a slab or cast strip having the following composition: are: 0.05 to 0.50% by weight of C; 0.50 to 8.0% by weight of Mn; 0.05 to 6.0% by weight of Al_tot; 0.0001 to 0.05% by weight of Sb; ∑ (Ca + REM) from 0.0005 to 0.005% by weight; 5 to 100 ppm N; 0 to 2.0% by weight of Si; 0 to 0.01% by weight of S; 0 to 0.1% by weight of P; 0 to 1.0% by weight of Cr; 0 - 2.0% by weight of Ni; 0 to 2.0% by weight of Cu; 0 to 0.5% Mo; V from 0 to 0.1% by weight; 0 to 50 ppm of B; 0 to 0.10% by weight of Ti; The component has a bimodal grain microstructure consisting of a ferrite matrix composed of delta-ferrite and alpha-ferrite, wherein the delta-ferrite has a grain size of 5 to 20 μm, and the alpha-ferrite has a grain size of 3 μm or less It has a grain size and has a second phase composed of one or more of bainite, martensite and retained austenite having a grain size of 3 μm or less. However, the steel of EP3421629 does not prove to be a low-density steel, but also contains hard surfaces such as martensite and bainite.

Accordingly, an object of the present invention is to provide a steel sheet exhibiting a relative density of less than 7.3, an ultimate tensile strength of 600 MPa or more, and a uniform elongation of 17.5% or more.

In a preferred embodiment, the steel sheet according to the present invention exhibits a relative density of 7.2 or less and a yield strength of 450 MPa or more.

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

The carbon content is between 0.12% and 0.25% by weight, more preferably between 0.13% and 0.2% by weight. Carbon is a Garmanius element that plays an important role in the formation of retained austenite and imparts strength and ductility. It is preferable that the carbon content is 0.13 to 0.2% to ensure high strength, elongation and stretch flangeability at the same time.

The manganese content is present between 3% and 10% by weight. Manganese is an important alloying element in this system, primarily due to the fact that alloying with very high amounts of manganese stabilizes austenite at room temperature, which can help reach target properties such as elongation and yield strength. Manganese, together with carbon, controls carbide formation at grain boundaries at high temperatures, thereby controlling high-temperature brittleness. The presence of more than 10% manganese can lead to central segregation which is detrimental to the ductility of the steels of this invention. The presence of less than 3% manganese will not stabilize the retained austenite in adequate amounts at room temperature. A preferred limit for the presence of manganese is between 4% and 9%, more preferably between 4% and 8%.

The aluminum content is present between 3.5% and 6.5% by weight. The aluminum added to the steel of this invention effectively reduces its density. Aluminum is an alpha genus element and therefore tends to promote the formation of ferrite and especially delta ferrite. Aluminum has a relative density of 2.7 and affects mechanical properties. As the aluminum content increases, the mechanical strength and elastic limit also increase, although the uniform elongation decreases due to the decrease in dislocation mobility. 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 adversely affects the present invention. In addition, the presence of more than 6.5% Al can form intermetallics such as Fe-Al, Fe ₃ -Al and other (Fe,Mn)Al intermetallics, which impart brittleness to the product during cold rolling. It can cause cracking of the steel and can also be detrimental to the toughness of the steel. Preferably, the aluminum content will be strictly limited to less than 6.5% to prevent the formation of brittle intermetallic precipitates, so the preferred limit is 4% to 6%, more preferably 5% to 6%.

Silicon can reduce the density of steel and is an effective optional element for solid solution hardening. Nevertheless, its content is limited to 2% by weight, since above that level this element tends to form strongly adhesive oxides which produce surface defects. The presence of surface oxides impairs the wettability of the steel and can potentially create defects during hot dip galvanizing operations. Therefore, it is desirable to limit the Si content to less than 1.5%.

Sulfur and phosphorus are impurities that embrittle grain boundaries. Their respective contents should not exceed 0.03% by weight and 0.1% by weight in order to maintain sufficient high temperature ductility.

The nitrogen content should be 0.1% by weight or less in order to prevent the formation of bulk defects (blisters) during precipitation and solidification of AlN.

Niobium may be added as an optional element to the steel of the present invention in an amount of 0.01% to 0.03% by weight to provide grain refinement. Grain refinement makes it possible to obtain a good balance between strength and elongation. However, niobium tends to retard recrystallization during hot rolling and annealing, so the limit is maintained at 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 the present invention for grain refinement in a manner similar to 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 steel and improve corrosion resistance. A minimum of 0.01% is required to achieve such an effect. However, if the copper content exceeds 2.0%, the surface appearance may deteriorate.

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 improve its toughness. A minimum of 0.01% is required to achieve such an effect. However, when the content of nickel exceeds 3.0%, nickel causes ductility deterioration.

Molybdenum is any element present in the steel of the present invention at 0% to 0.5% by weight. Molybdenum plays an effective role in improving hardenability and hardness when added at 0.01% or more. Mo also aids in the toughness of hot-rolled products, making them easier to manufacture. However, since the addition of molybdenum excessively increases the cost of adding an alloying element, its content is limited to 0.5% for economic reasons. Preferred limits for molybdenum are between 0% and 0.4%, more preferably between 0% and 0.3%.

Chromium is an optional element in the steel of the present invention and is between 0% and 0.6% by weight. Chromium provides strength and hardenability to the steel, but damages the surface finish of the steel when used in excess of 0.5%. A preferred limit for chromium is between 0.01% and 0.5%, more preferably between 0.01% and 0.2%.

Other elements such as cerium, boron, magnesium or zirconium may be added individually or in combination in the following weight ratios: Ce ≤ 0.1%, B ≤ 0.01, Ca ≤ 0.005, Mg ≤ 0.005 and Zr ≤ 0.005. Up to the indicated maximum content level, these elements make it possible to refine ferrite grains during solidification.

Also, some trace elements such as Sb and Sn may come from steel processing. The maximum limit at which these elements are permissible and which are not detrimental to the steel of the present invention is 0.05% by weight either cumulatively or alone, and it is preferred that the content of these elements be as low as possible and preferably less than 0.03% with the steel of the present invention. .

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

The delta ferrite matrix exists as a primary phase in the steel of the present invention, and accounts for 60% to 90% area fraction, preferably 65% to 90% area fraction, more preferably 80% to 90% area fraction in the steel of the present invention. exists as Delta ferrite is formed from liquid iron during solidification of the slab and generally has a coarse grain size. The delta ferrite of the present invention preferably has an average grain size of less than 10 μm, more preferably less than 9 μm. The presence of the delta ferrite matrix in the present invention imparts strength to the steel. However, the presence of a delta ferrite content of more than 90% in the present invention may have negative effects due to the fact that ferrite increases with increasing temperature solubility of carbon. However, solid solution carbon is highly brittle for low-density steels because the presence of aluminum reduces the mobility of the already low dislocations. Therefore, the balance between delta ferrite content and austenite is very important in imparting the mechanical properties required for the present invention.

Retained austenite is present at 8 to 30% in the steel of the present invention, wherein the retained austenite of the present invention has an average grain size of 0.6 microns to 2 microns. The preferred average grain size of retained austenite is between 0.6 microns and 1.2 microns. Retained austenite is known to act as an effective carbon trap due to its high carbon solubility compared to ferrite. The carbon percentage in austenite is between 0.7% and 1.5% by weight. Austenite present at levels above 30% adversely affects the present invention by impairing elongational flangeability. Austenite contributes to the present invention in very different ways depending on the choice of composition and annealing temperature of the steel, and the austenite of the present invention exhibits various functions such as providing formability and ductility by the TRIP effect. Preferred limits for retained austenite are between 9% and 29% as area fraction.

The alpha-ferrite of the present invention is present in an area fraction of 1% to 10%. Alpha ferrite is produced by partial transformation of austenite after critical annealing and during cooling after hot rolling, and has an average grain size of 0.6 to 1.85 microns. The preferred average grain size of alpha-ferrite is 0.6 microns to 1.2 microns. The alpha ferrite of the present invention imparts ductility and elongation. A preferred limit for alpha-ferrite is between 2% and 10% as an area fraction.

In the present invention, kappa precipitates are defined by precipitates having a stoichiometry of (Fe,Mn) ₃ AlCx, where x is strictly lower than 1. The area fraction of kappa precipitates can be up to 2%. Above 2%, ductility decreases and uniform elongation above 17.5% is not achieved. Also, uncontrolled precipitation of kappa around the ferrite grain boundaries may occur, and thus the effort during hot and/or cold rolling may increase. Preferentially, the area fraction of kappa precipitates should be less than 1%.

In addition to the microstructure described above, the microstructure of the low-density cold rolled and annealed steel is free from microstructure components such as pearlite, bainite and martensite.

The steel sheet according to the present invention can be produced by any suitable manufacturing method, which a person skilled in the art can define. However, preference is given to using the method according to the invention comprising the following steps:

The steel sheet according to the present invention is preferably obtained by casting a semi-finished product such as a slab, thin slab or strip made of the steel according to the present invention having the above-described composition, first cooling the cast raw material to room temperature, It is preferably prepared by reheating to a temperature of 1150 ° C. or more, more preferably 1200 ° C. or more, or by using a cast semi-finished product directly at such a temperature without intermediate cooling. Semi-finished products for this process are considered as slabs.

The reheated slab is then hot rolled. The hot rolling finishing temperature should be 750°C or higher, preferably 770°C or higher.

After hot rolling, the strip should be coiled at a temperature below 720°C, preferably between 350°C and 720°C, more preferably between 700°C and 400°C.

After the hot-rolled strip is cooled to room temperature, pickling or other scale removal processes are performed.

Then, the hot-rolled strip is cold-rolled at a reduction of 30% to 90%, preferably 40% to 90%.

After cold rolling, the cold-rolled steel sheet is heated at a heating rate of at least 1 °C/s, preferably at least 3 °C/s, from 840 °C to 1000 °C, preferably from 850 °C to 975 °C, more preferably from 850 °C to 925 °C. Heat the steel sheet to an annealing temperature of °C, hold the steel sheet at this annealing temperature for less than 1000 seconds, preferably less than 600 seconds, at least 3°C/s, more preferably at least 5°C/s, even more preferably is annealed by cooling at a rate of at least 10 °C/s. Preferably, this annealing is performed continuously.

A two-phase structure can be obtained during soaking by adjusting the annealing temperature and time.

After this annealing step, the steel sheet may be cooled to a temperature between room temperature and 480°C, optionally maintained at 100°C to 480°C, and overaged for less than 1 hour, preferably less than 20 minutes, more preferably less than 10 minutes. there is. After that, it can be cooled to room temperature.

After annealing, the steel sheet may optionally be subjected to a metal coating operation to improve its protection against corrosion. The coating process used may be any process suitable for the steels of the present invention. Electrolytic or physical vapor deposition may be mentioned, with particular emphasis on Jet Vapor Deposition. For example, the metallic coating may be based on zinc or aluminum.

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 balance being Al.

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

yes

The following tests, examples, figurative examples and tables presented herein are non-limiting in nature and are to be considered for illustrative purposes only, and will show the advantageous features of the present invention.

Steel plates made of steels with different compositions are collected in Table 1, where the presence of phosphorus is always less than 100 ppm for all steels, wherein the steel plates are each produced according to the process parameters as specified in Table 2. After that, Table 3 shows the microstructure of the steel sheets obtained during the test, and Table 4 shows the evaluation results of the properties obtained.

Table 1 - Composition

Table 2 - Process parameters

Then, the final samples were analyzed, and the corresponding microstructure elements and mechanical properties are listed in Tables 3 and 4, respectively.

Table 3 shows the results of tests performed according to standards on different microscopes, such as EBSD, XRD or other microscopes, to determine the microstructural composition of both the inventive steel and the reference tests. The area fractions of delta ferrite and alpha-ferrite are measured using EBSD. For a given steel sample, EBSD analysis of at least 4 images corresponding to a magnification of 1000 allows to identify the ferrite particles, their location and size. All grains having a grain size below the cut-off value of 1.85 μm and adjacent to austenite grains are counted as alpha ferrite, and the corresponding area fraction of these grains is determined. The remaining ferrite grains are counted as delta ferrite and the corresponding area fraction of these grains is determined. The average grain sizes of delta ferrite, retained austenite and alpha-ferrite are also determined using EBSD. The retained austenite area fraction is measured using XRD shown in Table 3.

Table 3

I = according to the present invention; R = reference; Underlined values: not according to the present invention

It can be seen from the table above that the tests according to the present invention all met the microstructural goal.

Table 4 discloses the mechanical and surface properties of both the inventive steel and the reference steel.

Table 4: Mechanical properties of the test

Yield strength (YS), tensile strength (TS) and uniform elongation (UE) are measured according to ISO standard ISO 6892-1 published in October 2009.

To determine the relative density of 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 the other side. The mass-per-volume ratio of steel in g/cm ³ can then be calculated and further divided by the mass-per-volume ratio of water at 4° C., which equals 1 g/cm ³ . The unitless result is the relative density of the steel.

The examples show that the steel sheets according to the invention are unique, exhibiting all the target properties thanks to their specific composition and microstructure.

Claims (14)

As a low-density cold-rolled and annealed steel sheet, by weight,
0.12% ≤ carbon ≤ 0.25%;
3% ≤ manganese ≤ 10%;
3.5% ≤ aluminum ≤ 6.5%;
0% ≤ Phosphorus ≤ 0.1%;
0% ≤ sulfur ≤ 0.03%;
0% ≤ nitrogen ≤ 0.1%;
and optionally, one or more of the following elements:
0% ≤ silicon ≤ 2%;
0.01% ≤ niobium ≤ 0.03%;
0.01% ≤ titanium ≤ 0.2%;
0% ≤ molybdenum ≤ 0.5%;
0% ≤ chromium ≤ 0.6%;
0.01% ≤ copper ≤ 2.0%;
0.01% ≤ Nickel ≤ 3.0%;
0% ≤ calcium ≤ 0.005%;
0% ≤ boron ≤ 0.01%;
0% ≤ Magnesium ≤ 0.005%;
0% ≤ zirconium ≤ 0.005%;
0% ≤ cerium ≤ 0.1%;
The remainder contains iron and unavoidable impurities, and the steel sheet contains, in area fraction, 60 to 90% delta ferrite, 8% to 30% retained austenite with an average grain size of 0.6 to 2 microns, 0.6 to 1.85 microns. 1.0% to 10% of alpha-ferrite with an average grain size of 0% to 2% of kappa precipitates (Fe,Mn) ₃ AlC _x with x strictly lower than 1, with a microstructure comprising annealed steel sheet. The low-density cold-rolled and annealed steel sheet according to claim 1, wherein the carbon content is 0.13% to 0.2%. The low-density cold-rolled and annealed steel sheet according to claim 1 or 2, wherein the manganese content is 4% to 9%. The low-density cold rolled and annealed steel sheet according to any one of claims 1 to 3, wherein the retained austenite content is 9% to 29%. The low-density cold-rolled and annealed steel sheet according to any one of claims 1 to 4, wherein the kappa precipitate content is 0% to 1%. 6. Low density cold rolled and annealed steel sheet according to any one of claims 1 to 5, wherein the alpha-ferrite content is 2% to 10% with an average grain size of 0.6 to 1.2 microns. The low-density cold-rolled and annealed steel sheet according to any one of claims 1 to 6, wherein the steel sheet is covered with a metal coating. As a method for producing a steel sheet, the following steps:
- supplying a slab having a composition according to claims 1 to 3;
- reheating the slab at a temperature above 1000 °C and hot rolling the slab to a final rolling temperature of at least 750 °C,
- coiling the hot-rolled steel sheet at a temperature of less than 720 ° C,
- cooling the hot-rolled steel sheet;
- pickling the hot-rolled steel sheet;
- cold rolling the hot-rolled steel sheet at a reduction ratio of 30 to 90% to obtain a cold-rolled steel sheet;
- annealing the cold-rolled steel sheet by heating the steel sheet from room temperature to an annealing temperature of 840° C. to 1000° C. at a heating rate of at least 1° C./s;
- then carrying out annealing for less than 1000 seconds,
- then cooling the cold rolled steel sheet to a cooling stop temperature of 480°C to room temperature at a cooling rate of at least 3°C/s, optionally maintaining the cold rolled steel sheet at 100°C to 480°C for 1 to 200 seconds step to do,
- Then, cooling the cold-rolled steel sheet to room temperature to obtain a low-density cold-rolled and annealed steel sheet. The method according to claim 8, wherein the annealing temperature is 850 °C to 975 °C. The method according to any one of claims 8 and 9, wherein the coiling temperature is 350 °C to 720 °C. The method for manufacturing a steel sheet according to any one of claims 8 to 10, wherein the holding time of annealing is less than 600 seconds. The method for producing a steel sheet according to any one of claims 8 to 11, wherein the heating rate for annealing is greater than 3 °C/s. The method for producing a steel sheet according to any one of claims 8 to 12, further comprising a final coating step. Use of a steel sheet according to any one of claims 1 to 7 or obtainable according to a method according to any one of claims 8 to 14, for the manufacture of structural or safety parts for vehicles.