Patent Grant
9580766
The invention relates to hot-rolled or cold-rolled ferritic steel sheet possessing a strength of greater than 400 MPa and a density of less than about 7.3, and to its manufacturing process.
The quantity of CO2 emitted by motor vehicles can be reduced in particular by lightening said motor vehicles. This lightening may be achieved by:
The first approach has been the subject of extensive research, steels having been proposed by the steel industry that have a strength ranging from 800 MPa to more than 1000 MPa. The density of these steels however remains close to 7.8, which is the density of conventional steels.
A second approach involves the addition of elements capable of reducing the density of the steels. Patent EP 1 485 511 thus discloses steels having additions of silicon (2-10%) and aluminium (1-10%), with a ferritic microstructure, and also containing carbide phases.
However, the relatively high silicon content of these steels may in certain cases pose coatability and ductility problems.
Also known are steels containing an addition of about 8% aluminium. However, difficulties may be encountered when manufacturing these steels, in particular during cold rolling. Roping problems may also be encountered when drawing these steels. When such steels contain more than 0.010% C, the precipitation of carbide phases may increase brittleness. The use of such steels for manufacturing structural parts is then impossible.
One object of the invention is to provide hot-rolled or cold-rolled steel sheet having, simultaneously:
Another object of the invention is to provide a manufacturing process compatible with the usual industrial installations.
For this purpose, one subject of the invention is a hot-rolled ferritic steel sheet, the composition of the steel of which comprises, the contents being expressed by weight: 0.001≦C≦0.15%, Mn≦1%, Si≦1.5%, 6%≦Al≦10%, 0.020%≦Ti≦0.5%, S≦0.050%, P≦0.1% and, optionally, one or more elements chosen from: Cr≦1%, Mo≦1%, Ni≦1%, Nb≦0.1%, V≦0.2%, B≦0.01%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the average ferrite grain size dIV measured on a surface perpendicular to the transverse direction with respect to the rolling being less than 100 microns.
Another subject of the invention is a cold-rolled and annealed ferritic steel sheet, the steel of which has the above composition, characterized in that its structure consists of equiaxed ferrite, the average grain size dα, of which is less than 50 microns, and in that the linear fraction f of intergranular κ precipitates is less than 30%, the linear fraction f being defined by
denoting the total length of the grain boundaries containing κ precipitates relative to an area (A) in question and
denoting the total length of the grain boundaries relative to said area (A) in question.
According to one particular embodiment, the composition comprises: 0.001%≦C≦0.010%, Mn≦0.2%.
According to a preferred embodiment, the composition comprises: 0.010%<C≦0.15%, 0.2%<Mn≦1%.
Preferably, the composition comprises: 7.5%≦Al≦10%.
Very preferably, the composition comprises: 7.5%≦Al≦8.5%.
The content of carbon in solid solution is preferably less than 0.005% by weight.
According to a preferred embodiment, the strength of the sheet is equal to or greater than 400 MPa.
Preferably, the strength of the sheet is equal to or greater than 600 MPa.
Another subject of the invention is a process for manufacturing a hot-rolled steel sheet in which: a steel composition according to one of the above compositions is supplied; the steel is cast in the form of a semi-finished product; then said semi-finished product is heated to a temperature of 1150° C. or higher; then the semi-finished product is hot-rolled so as to obtain a sheet using at least two rolling steps carried out at temperatures above 1050° C., the reduction ratio of each of the steps being equal to or greater than 30%, the time elapsing between each of the rolling steps and the next rolling step being equal to or greater than 10 s; then the rolling is completed at a temperature TER of 900° C. or higher; then the sheet is cooled in such a way that the time interval tp elapsing between 850 and 700° C. is greater than 3 s so as to cause the precipitation of κ precipitates; and then the sheet is coiled at a temperature Tcoil between 500 and 700° C.
According to one particular method of implementation, the casting is carried out directly in the form of thin slab or thin strip between counter-rotating rolls.
Another subject of the invention is a process for manufacturing a cold-rolled and annealed steel sheet, in which: a hot-rolled steel sheet manufactured according to one of the above methods is supplied; then the sheet is cold-rolled with a reduction ratio between 30 and 90% so as to obtain a cold-rolled sheet; then the cold-rolled sheet is heated to a temperature T′ at a rate Vh greater than 3° C./s; and then the sheet is cooled at a rate Vc less than 100° C./s, the temperature T′ and rate Vc being chosen so as to obtain complete recrystallization, a linear fraction f of intergranular κ precipitates of less than 30% and a content of carbon in solid solution of less than 0.005% by weight.
Preferably, the cold-rolled sheet is heated to a temperature T′ between 750 and 950° C.
According to one particular method of manufacturing a cold-rolled and annealed sheet, a sheet is supplied with the following composition: 0.010%<C≦0.15%; 0.2%<Mn≦1%; Si≦1.5%; 6%≦Al≦10%; 0.020%≦Ti≦0.5%; S≦0.050%; P≦0.1% and, optionally, one or more elements chosen from: Cr≦1%, Mo≦1%, Ni≦1%, Nb≦0.1%, V≦0.2%, B≦0.01%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, and the cold-rolled sheet is heated to a temperature T′ chosen so as to avoid the dissolution of κ precipitates.
According to one particular method of implementation, a sheet of the above composition is supplied and the cold-rolled sheet is heated to a temperature T′ between 750 and 800° C.
Another subject of the invention is the use of steel sheet according to one of the above embodiments or manufactured according to one of the above methods for the manufacture of skin parts or structural parts in the automotive field.
Other features and advantages of the invention will become apparent over the course of the description below, given by way of example and with reference to the figures appended herewith, in which:
The present invention relates to steels having a reduced density, of less than about 7.3, while maintaining satisfactory usage properties.
The invention relates in particular to a manufacturing process for controlling the precipitation of intermetallic carbides, the microstructure and the texture in steels containing especially particular combinations of carbon, aluminium and titanium.
As regards the chemical composition of the steel, carbon plays an important role in the formation of the microstructure and in the mechanical properties.
According to the invention, the carbon content is between 0.001% and 0.15%. Below 0.001%, significant hardening cannot be obtained. When the carbon content is above 0.15%, the cold rollability of the steels is poor.
When the manganese content exceeds 1%, there is a risk of stabilizing the residual austenite at ambient temperature because of the propensity of this element to form the gamma-phase. The steels according to the invention have a ferritic microstructure at ambient temperature. Various particular methods of implementing the invention may be employed, depending on the carbon and manganese contents of the steel:
Within the carbon content ranges presented above, the inventors have demonstrated that this element contributes to substantial hardening by the precipitation of carbides (TiC or kappa precipitates) and by ferrite grain refinement. The addition of carbon results in only a small loss of ductility if the carbide precipitation is not intergranular or if the carbon is not in solid solution.
Within these composition ranges, the steel has a ferrite matrix at all temperatures during the manufacturing cycle, that is to say right from solidification after casting.
Like aluminium, silicon is an element allowing the density of the steel to be reduced. However, an excessive addition of silicon, above 1.5%, results in the formation of highly adherent oxides and the possible appearance of surface defects, leading in particular to a lack of wettability in hot-dip galvanizing operations. Furthermore, this excessive addition reduces the ductility.
Aluminium is an important element in the invention. When its content is less than 6% by weight, a sufficient reduction in density cannot be obtained. When its content is greater than 10%, there is a risk of forming embrittling intermetallic phases Fe3Al and FeAl.
Preferably, the aluminium content is between 7.5 and 10%. Within this range, the density of the sheet is less than about 7.1.
Preferably, the aluminium content is between 7.5 and 8.5%. Within this range, satisfactory lightening is obtained without a reduction in ductility.
The steel also contains a minimal amount of titanium, namely 0.020%, which helps to limit the content of carbon in solid solution to an amount of less than 0.005% by weight, thanks to the precipitation of TiC. Carbon in solid solution has a deleterious effect on the ductility because it reduces the mobility of dislocations. Above 0.5% titanium, excessive titanium carbide precipitation takes place, and the ductility is reduced.
An optional addition of boron, limited to 0.010%, also helps to reduce the amount of carbon in solid solution.
The sulphur content is less than 0.050% so as to limit any precipitation of TiS, which would reduce the ductility.
For hot ductility reasons, the phosphorus content is also limited to 0.1%.
Optionally, the steel may also contain, alone or in combination:
The balance of the composition consists of iron and inevitable impurities resulting from the smelting.
The structure of the steels according to the invention comprises a homogeneous distribution of highly disoriented ferrite grains. The strong disorientation between neighbouring grains prevents the roping defect. This defect is characterized, during cold-forming of sheet, by the localized and premature appearance of strip in the rolling direction, forming a relief. This phenomenon is due to the grouping of recrystallized grains that are slightly disoriented, as they come from one and the same original grain before recrystallization. A structure sensitive to roping is characterized by a spatial distribution in the texture.
When the roping phenomenon is present, the mechanical properties in the transverse direction (especially the uniform elongation) and the formability are greatly reduced. The steels according to the invention are insensitive to roping during forming, because of their favourable texture.
According to one embodiment of the invention, the microstructure of the steels at ambient temperature consists of an equiaxed ferrite matrix, the average grain size of which is less than 50 microns. The aluminium is predominantly in solid solution within this iron-based matrix. These steels contain kappa (κ) precipitates, which are an Fe3AlCx ternary intermetallic phase. The presence of these precipitates in the ferrite matrix results in substantial hardening. These κ precipitates must not however be present in the form of pronounced intergranular precipitation, as otherwise there would be a substantial reduction in ductility. The inventors have demonstrated that the ductility is reduced when the linear fraction of ferrite grain boundaries in which there is κ precipitation is equal to or greater than 30%. The definition of this linear fraction f is given in
denoting the total length of the grain boundaries containing κ precipitates relative to the area (A) in question and
denoting total length of the grain boundaries relative to the area (A) in question. The expression f therefore represents the degree to which the ferrite grain boundaries are covered with κ precipitates.
According to another embodiment, the ferrite grain is not equiaxed but its average size dIV is less than 100 microns. The term dIV denotes the grain size measured by the method of linear intercepts over a representative area (A) perpendicular to the transverse direction with respect to rolling. The dIV measurement is carried out along the direction perpendicular to the thickness of the sheet. This non-equiaxed grain morphology, having an elongation in the rolling direction, may for example be present on hot-rolled steel sheets according to the invention.
The method of implementing the process for manufacturing a hot-rolled sheet according to the invention is the following:
The cast semi-finished products are firstly heated to a temperature above 1150° C. so as to achieve, at all points, a temperature favourable to large deformations that the steel will undergo during the various rolling steps.
Of course, in the case of direct thin slab or thin strip casting between counter-rotating rolls, the step of hot rolling these semi-finished products starting at above 1150° C. may be carried out directly after casting, so that an intermediate reheating step is in this case unnecessary.
After many trials, the inventors have demonstrated that it is possible to prevent the problem of roping and to obtain very good drawability and good ductility, by means of the manufacturing process comprising the following steps:
The inventors have also demonstrated that a fine initial structure, like that obtained after direct casting, is favourable to increasing the rate of recrystallization;
At this stage, a hot-rolled sheet is thus obtained that has a thickness of for example 2 to 6 mm. If it is desired to manufacture a sheet of smaller thickness, for example 0.6 to 1.5 mm, the manufacturing process is the following:
The sheet is then cooled at a rate Vc of less than 100° C./s so as not to cause any embrittlement by excess carbon in solid solution. This result is particularly surprising in so far as it might be considered that a rapid cooling rate would be favourable to reducing embrittling precipitation. Now, the inventors have demonstrated that slow cooling, at a cooling rate of less then 100° C./s, results in substantial carbide precipitation which thus reduces the content of carbon in solid solution. This precipitation has the effect of increasing the strength without a deleterious effect on the ductility.
The annealing temperature T′ and the rate Vc will be chosen so as to obtain, on the final product:
A temperature T′ between 750 and 950° C. will be preferably chosen so as to obtain complete recrystallization. More particularly, when the carbon content is greater than 0.010% but less than or equal to 0.15%, and when the manganese content is greater than 0.2% but less than or equal to 1%, the temperature T′ will be chosen so as to furthermore prevent dissolution of the κ precipitates present before annealing. This is because, if these precipitates have dissolved, the subsequent precipitation on slow cooling will take place in embrittling intergranular form: too high an annealing temperature will result in redissolution of the κ precipitates formed during manufacture of the hot-rolled sheet and reduce the mechanical strength. For this purpose, it is preferable to choose a temperature T′ between 750 and 800° C.
By way of non-limiting example, the following results will show the advantageous properties conferred by the invention.
Steels were produced by casting them in the form of semi-finished products with a thickness of about 50 mm. Their compositions, expressed in percentages by weight, are given in Table 1 below.
R1
0.006
R2
1.21
—
R3
14.5
R4
0.19
1.45
12.6
R5
0.197
1.7
10.2
—
R6
0.19
12.2
2.2
The semi-finished products were reheated to a temperature of 1220° C. and hot rolled to obtain a sheet with a thickness of about 3.5 mm.
Starting from the same composition, some of the steels were subjected to various hot-rolling conditions. The references I1-a, I1-b, I1-c, I1-d and I1-e denote for example five steel sheets manufactured under different conditions from the composition I1.
In the case of steels I1 to I3, Table 3 details the conditions for the successive hot-rolling steps:
I1b
2
2
I1c
1
8
I1e
3
I3b
1
5
Table 3 shows the measured density on the sheets of Table 2 and certain mechanical and microstructural properties. Thus, the following were measured, in the transverse direction with respect to rolling: the strength Rm, the uniform elongation Au and the elongation at break At. Also measured was the grain size dIV using the method of linear intercepts according to the NF EN ISO 643 standard of a surface perpendicular to the transverse direction with respect to rolling. The dIV measurement was carried out along the direction perpendicular to the thickness of the sheet. For the purpose of obtaining enhanced mechanical properties, a grain size dIV of less than 100 microns is more particularly sought.
I1b
200
I1c
450
I1e
120
I3b
400
The steel sheets according to the invention, the microstructure of which is illustrated for example in
Sheets I1b and I1e were rolled with too short an inter-pass time. Their structure is therefore coarse and non-recrystallized or insufficiently recrystallized, as shown in
Sheet I1c was rolled with an insufficient number of rolling steps with a reduction ratio greater than 30%, too short an inter-pass time and too short a time interval tp. The consequences are the same as those noted in the case of sheets I1b and I1e. Since the time interval tp is too short, hardening precipitation of κ precipitates and TiC carbides takes place only partially, thereby making it impossible to take full advantage of the hardening possibilities.
The semi-finished products produced from the reference steels R1 to R6 were rolled so as to manufacture hot-rolled sheets under manufacturing conditions identical to those of steel I3a of Table 2. The properties obtained on these sheets are given in Table 4.
7.44
0.1
0.1
0.6
0.6
2.7
2.7
0.7
0.7
Steel R1 possesses an insufficient titanium content, thereby leading to too high a content of carbon in solid solution—the bendability is therefore reduced.
Steel R2 possesses an insufficient aluminium content, thereby preventing a density of less than 7.3 being obtained.
Steels R3, R4, R5 and R6 contain too high an amount of aluminium and possibly of carbon. Their ductility is reduced because of excessive precipitation of intermetallic phases or carbides.
Starting from hot-rolled steel sheets I1-a and I3-a (according to the invention) and I1-c and I3-b (not complying with the conditions of the invention), a cold-rolling operation was carried out with a reduction ratio of 75% in order to obtain sheets with a thickness of about 0.9 mm. The cold-rollability was noted during this step. Next, an annealing operation was carried out, characterized by a heating rate Vh=10° C./s. The annealing temperatures T′ and the cooling rates Vc are given in Table 5. Under these conditions, the annealing results in complete recrystallization.
Starting from the same hot-rolled sheet, certain steels were subjected to various cold-rolling and annealing conditions. The references I3a1, I3a2, I3a3 and I3a4 denote for example four steel sheets manufactured under different cold-rolling and annealing conditions from the hot-rolled sheet I3a.
150° C./s
I1c1
I3a2
150° C./s
I3a3
900° C.
I3a4
900° C.
150° C./s
I3b
Unsatisfactory
Table 6 shows certain mechanical, chemical, microstructural and density properties of the sheets of Table 5. Thus, the yield strength Re, the tensile strength Rm, the uniform elongation Au and the elongation at break At were measured by tensile tests in the transverse direction with respect to rolling. The possible presence of cleavage facets on the fracture surfaces of the test specimens was revealed by scanning electron microscope observations.
The content of carbon in solid solution Csol was also measured, as were the bendability and drawability. The possible presence of roping following deformation was also revealed.
The microstructure of these recrystallized sheets consisted of equiaxed ferrite, the average grain size dα of which was measured in the transverse direction with respect to rolling. Also measured was the degree of coverage f of the ferrite grain boundaries with κ precipitates, by means of Aphelion™ image analysis software.
I1a2
Ductile/brittle
0.005
I1c1
53
Yes
No
I3a2
Ductile/brittle
0.010
I3a3
Ductile/brittle
60
No
I3a4
Brittle
0.060
No
Steel sheets I1a1 and I3a1 have a content of carbon in solid solution, an equiaxed ferrite grain size and a degree of coverage f of the grain boundaries that meet the conditions of the invention. Consequently, the bendability, the drawability and the roping resistance of these sheets are high.
In comparison, steel sheet I1a2 was cooled at too high a rate after annealing: the carbon is then completely in solid solution, resulting in a reduction in ductility of the matrix manifested by the local presence of brittle areas on the fracture surfaces. Likewise, sheet I3a2 was cooled at too high a rate and also results in an excessive content in solid solution.
Sheet I3a4 was also annealed at a temperature resulting in partial dissolution of the κ precipitates. The content of carbon in solid solution is excessive.
Steel sheet I1c1 was manufactured from a hot-rolled sheet not complying with the conditions of the invention: the equiaxed grain size was too high, and the roping resistance and drawability were insufficient.
Hot-rolled sheet I3b, not meeting the criteria of the invention, is incapable of deformation since transverse cracks appear during cold rolling.
Spot resistance weldability trials were carried out on steel sheet I1a1, either in homogeneous welding (welding of two sheets of the same composition) or heterogeneous welding (welding with an interstitial-free steel sheet of the following composition, expressed in percentages by weight: 0.002% C, 0.01% Si; 0.15% Mn; 0.04% Al; 0.015% Nb; and 0.026% Ti). Examinations of the welded joints showed that they were defect-free.
In the case of a subsequent heat treatment of the welded joints, the addition of 0.096% Ti guarantees the absence of carbon in solid solution in the heat-affected zone.
The steels according to the invention exhibit good continuous galvanizability, in particular during an annealing cycle at 800° C. with a dew temperature above −20° C.
The steels according to the invention therefore have a particularly advantageous combination of properties (density, mechanical strength, deformability, weldability, coatability). These steel sheets are used to advantage for the manufacture of skin or structural parts in the automotive field.
| Number | Date | Country | Kind |
|---|---|---|---|
| 072906241 | May 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2008/000610 | 4/29/2008 | WO | 00 | 3/2/2010 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2008/145872 | 12/4/2008 | WO | A |
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| Number | Date | Country | |
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| 20100300585 A1 | Dec 2010 | US |
