Patent Application
20220002847
The present invention relates to a method for providing a medium-manganese cold strip steel intermediate product with reduced carbon content and medium-manganese cold strip steel intermediate products with reduced carbon content.
Both the composition, respectively alloy as well as the heat treatment in the manufacturing process do have a significant influence on the properties of steel products.
A major component of today's steel-alloys is manganese (Mn). The content of manganese in weight % is often in the range between 3 and 12%. These steels are therefore so-called median-manganese steels, which are also referred to as a medium-manganese steels.
Medium manganese steels are characterized, for example, by a structure that consists of a ferritic matrix and retained austenite. The content of ferrite in medium manganese steels usually has a maximum at 90 volume %. The austenite content, however, is usually in the range of about 30 vol. %.
Ferrite (also alpha- or α-mixed crystal) is the metallurgic designation of a body-centered cubic iron mixed crystal, in the lattice of which carbon (i.e., in intermediate positions of the lattice) is dissolved interstitially. A pure ferritic structure possesses a low strength but a high ductility. The strength can be improved by adding carbon, whereby this is at the expense of the ductility.
An austenite structure (also called gamma- or γ-mixed crystal) is a face-centered cubic iron mixed crystal which can form in a steel product. This is a high-temperature phase which can be stabilized at room temperature by the addition of alloying elements, such as, for example, carbon, manganese, nickel, etc.
Over the years there have been several development stages or advancements in the field of medium manganese steels.
In
In the area which is designated by the reference number 1, the medium-manganese steels mentioned above are summarized schematically. The area denoted by reference number 1 comprises medium manganese steels with an Mn content between 3 and 12 weight %.
The so-called TRIP steels are designated by the reference number 2 and the so-called TRIP bainitic ferrite (TBF) and the Quenching and Partitioning (Q&P) steels carry the reference number 3. TRIP stands in English for “Transformation Induced Plasticity”.
In the automotive sector one is working with a whole range of different cold formable steel alloys which were each optimized for their respective application in the vehicle. Alloys with good energy absorption are used in interior and exterior panels, structural parts and bumpers. Alloys for the outer skin of a vehicle have a lower yield strength and tensile strength typically up to 600 MPa and a higher elongation after fracture. The steel alloys of structural components, for example, have a tensile strength in the range between 600 and 1200 MPa. For example, the TRIP steels are suitable for this (reference number 2 in
In the meantime, there are medium-manganese steels, which belong to the 3rd generation of the Advanced High Strength Steels (AHSS). These steels show a good combination of strength and elongation. Newer steels of the 3rd generation achieve RmxA80 values of approx. 30,000 MPa % and are therefore suitable, for example, for the production of complex deep-drawn components such as those used in the automotive industry (reference number 1 in
On the process side, there are many different ways of making such powerful steels, inter alia, the temperature ranges which are specified, the heating and cooling rates and other aspects have a huge impact on the structure and hence on the quality and characteristics of the steel product.
There is a need to provide cold strip steel intermediate products, which have an improved deformability as compared to the known cold strip steel intermediate products. The deformability consists of a global and a local part. The global deformability primarily describes the behavior of the material during deep-drawing operations. The uniform elongation Ag, in English uniform elongation (UE), is suitable for describing the global deformability. The local deformability on the other hand, is a measure of the behavior of the material under multiaxial stress conditions, as for example occur in a hole expansion test. The fracture thickness strain in percent, abbreviated to fts, is a corresponding measure of the local deformability of steels. A detailed description of this characteristic can be found in P. Larour et al., “Reduction of cross section area at fracture in tensile test: measurement and applications for flat sheet steels”, IDDRG 2017.
So far, a compromise had to be found mostly between the local and global deformability. DP steels (DP steel stands for Dual-phase steel) have significantly lower fts values than CP steels (CP-steel stands for complex-phase steel), as can be derived from the graph of
There are several reasons for the different properties of DP steels and CP steels. One reason is the different hardness contrasts between the individual structural components of these materials. DP steels typically have a high hardness contrast of the structure compared to CP steels. The DP steels therefore show a high hardening rate and thus high elongation, i.e. high UE values. DP steels are not well locally deformable, but can be deepdrawn well. CP steels, on the other hand, harden less than DP steels and can therefore be locally deformed better.
Medium-manganese steels, at issue here, show because of their structure a similarly high hardness contrast as the DP steels, therefore, here a better global formability, i.e. higher UE values, are to be expected. The high hardness contrast in medium manganese steels results from the transformation of residual austenite into hard martensite during deformation. This leads to high hardness contrasts between the soft ferritic matrix and hard martensitic inclusions.
In particular, the object is to provide cold strip steel intermediate products which have a good combination of tensile strength and elongation after fracture and which at the same time show a good local deformability. It arises in particular the object to provide cold strip steel intermediate products which have a better combination of uniform elongation (expressed as UE values) and local deformability (expressed in fts values) as DP- and CP-steels.
A cold strip steel intermediate product is provided whose structure includes a low martensitic strength, the highest possible ferritic strength and possibly a homogeneous and slowly transforming austenite, because of the high stability.
A method for providing a medium-manganese cold strip steel intermediate product is claimed, the alloy of which comprises:
In at least part of the embodiments, the intercritical box annealing is selected as part of a one-step annealing process so that the cold strip steel intermediate product after this step has a microstructure with the following proportions:
In at least part of the embodiments, the intercritical box annealing method is selected as part of a two-step annealing process so that the cold strip steel intermediate product after this step has a microstructure with the following proportions:
In at least some of the embodiments, an annealing temperature is specifically chosen which is dependent on the carbon content in wt. % and which is lower than the maximum annealing temperature in order to obtain a medium-manganese-cold-strip steel intermediate product that has an fts-value that is at least 40%. If a one-step annealing process is used, the maximum annealing temperature is defined by the formula 648° C.−(352° C.*the carbon content in wt. %). If a two-step annealing process is used, the maximum annealing temperature is defined by the formula 684° C.−(517° C.*the carbon content in wt. %).
In accordance with the invention a steel intermediate product having a good local and global good formability, preferably a cold strip steel intermediate product, is provided by a combination of a process- and an alloying-concept.
According to the invention, a cold strip steel intermediate product is provided that has a good Rm*Aso combination, as with other medium-Mangan steels, and at the same time a good local deformability, i.e. high fts values.
Such cold strip steel intermediate products are provided by the inventive method in that the carbon content is lowered and the Ferrite morphology, respectively Austenite morphology are intentionally changed by a specially adapted annealing. Furthermore, a residual austenite with high stability is adjusted by lowering the intercritical annealing temperature which is applied during manufacturing in annealing the steel intermediate product.
Although one typically increases the carbon content, if an increased strength steel intermediate product is wanted, the invention relies on a significant reduction of the carbon content. By reducing the carbon content a lower martensitic strength is achieved, which corresponds to a reduction of the hardness contrast it in the structure.
Although typically relatively high silicon- and aluminum-contents are employed, the invention uses a significant reduction of the silicon- and aluminum-contents. The silicon- and aluminum-alloy proportions are limited by the formula Si wt. %+Al wt. %<1. Since the silicon- and aluminum-alloy proportions are limited here, the annealing processes can be carried out with modified parameters.
In at least part of the embodiments one specially uses an alloy composition which comprises only a low sulfur content. The sulfur content is preferably less than 60 ppm. By reducing the sulfur content, fewer sulfides are formed and the fts values can improve, depending on the design of the annealing process.
Based on thermodynamic models, the optimum annealing temperature can be calculated for a steel alloy, which is chosen to achieve the maximum residual austenite content and thus an excellent combination of RmxA80.
The method of the invention is based on a specially optimized medium-manganese alloy, and is in addition based on a lower annealing temperature, since due to the lower temperature during the annealing better deforming properties are achieved. By lowering the intercritical annealing temperature, the medium-manganese alloy of the invention loses some of its tensile strength and uniform elongation, but simultaneously a higher residual austenite stability is reached, which leads to a higher global deformability (i.e. higher fts values).
In order to alter the Ferrite morphology, respectively the Austenite morphology specifically, in at least some of the embodiments of a fully austenitic annealing is applied, followed by an intercritical annealing. This results in higher fts values for the correspondingly annealed intermediate steel products.
Preferably the invention is used to provide cold strip steel intermediate products in the form of cold rolled flat products (for example, coils).
Exemplary embodiments of the invention are described in more detail below with reference to the drawings.
The cold strip steel intermediate products of the invention are produced by lowering the carbon content of the initial alloy. It has been shown that the fts value can be increased by significantly reducing the carbon content. By reducing the carbon content, the hardness contrast in the structure is reduced. This relationship has been confirmed and quantified on the basis of studies, which have shown that there are limits for the carbon content. In the context of the invention, only alloys are thus used whose carbon content is less than 0.12 wt. %.
The fts value is to be determined on a tested, non-notched steel flat tensile specimen. The initial thickness of the intermediate steel product d0 and the thickness at the fracture surface d1 must be determined. The fts value is calculated as follows (do−d1)/do*100 in %.
Different correlations can be derived from
Similar observations could also be made for alloys 2 and 3 (Leg. 2, Leg. 3 abbreviated).
Moreover, it was shown that the fts value with decreasing carbon content increases significantly. The Leg. 2 has the following composition:
The Leg. 3 has the following composition:
In other words, such a medium-manganese alloy should not be annealed too high and it should preferably have a low carbon content, if one wants to achieve high fts values. The block arrow designated with −C, which in
The lowering of the annealing temperature leads to a higher chemical enrichment of the austenite, to a smaller grain size, and a more stable residual austenite. Investigations have shown a residual austenite proportion which, in the case of the alloys of the invention, is advantageously in the range ≥10% and ≤60%. These effects lead to increased fts values.
The influence of various annealing methods on the resulting fts values have also been examined. In this context a 1st annealing route (GR 1 hereinafter) with an intercritical box annealing method (Method S.2.1 in
Those alloy samples which have been subjected to the 1st annealing route GR 1 with only one intercritical box annealing (Method S.2.1 in
The alloy samples which have been subjected to the 2nd annealing route GR 2 with a fully austenitic annealing followed by an intercritical box annealing method (Method S.1+S.2.2 in
If a double annealing GR 2 with a fully austenitic annealing step (S.1 method in
Further investigations of these alloy samples have shown that in comparison of a first alloy sample, which did pass through the 1st annealing route GR 1, and an identical second alloy sample, which did pass through the 2nd annealing route GR 2, the 2nd annealing route GR 2 also results in an increase in the uniform elongation UE. I.e., the choice of the annealing route and the parameters (holding temperatures H1 or H2, holding period Δ1 or Δ2, etc.) of the respective annealing routes not only have an influence on the fts-value but also an impact on the UE Value.
Alloy samples with the following compositions have been prepared here and have been subjected to the 1st annealing route GR 1 (see Table 1). For these alloys, tensile strengths Rm in the range between 663 MPa and 873 MPa could be achieved. The fts values of this alloy samples did range from about 48% to 74% and the UE-values did range from about 14% to 32%.
Alloy samples have been prepared here with the following compositions and have been subjected to the 2nd annealing route GR 2 (see Table 2). For these alloys, tensile strengths Rm in the range between 597 MPa and 996 MPa could be achieved. The fts values of these alloy specimens were in the range from about 51% to 75%, and the UE-values ranging from about 10% to 36%.
Table 3 provides the mechanical characteristic values as result of different temperature treatments. Tensile strengths in the range of 820 MPa and 875 MPa and uniform elongations in the range of 27% and 31% been achieved for the respective temperature treatments. The fts values achieved prove to be advantageous. A fully austenitic annealing S.1 as part of a 2-stage annealing procedure GR 2, according to
In summary, the following can be postulated for the examined alloy compositions of the invention:
In summary, for the investigated alloy compositions of the invention, the following can be postulated:
These postulates that were previously summarized in a simplified and purely schematic form, give the developer a number of degrees of freedom in the definition of alloys at hand. This will be illustrated by the following example.
When employing the double annealing (GR 2) one can work with alloys whose carbon content per se is somewhat higher than in the simple annealing GR 1, since with the double annealing (GR 2) higher fts values are achieved than with the simple annealing (GR 1).
In
The dotted line connecting the white diamonds represents the experimentally determined annealing temperatures TANMax is for alloys which were subjected a double annealing method (GR 2) were. The dashed line connecting the black squares represents the experimentally determined annealing temperatures TANmax for alloys that were subjected to a single annealing process (GR 1). The solid line connecting the white circles shows the experimentally determined annealing temperatures TRAmax when the maximum amount of retained austenite is reached as a function of the carbon content.
Alloy compositions which comprise 6 wt. % content of manganese (Mn) have been investigated here. The carbon content has been varied, as indicated on the abscissa, from 0 wt. % to 0.12 wt. %.
The dotted line in
T
ANmax=684° C.−(517° C.*C %) (1).
The dashed line in
T
ANmax=648° C.−(352° C.*C %) (2)
It was confirmed by the investigations, the results of which are summarized in
From the results summarized in
It can also be deduced from the results summarized in
If one anneals twice, as shown in
In
fts
min=104*e(−0.001*Rm) (3).
In
In Table 4 some characteristic properties of the alloys of the invention are summarized.
Some alloy compositions and their characteristic properties are summarized in Table 5. These alloy compositions combined with an annealing temperature chosen according to the invention are shown on purpose in Table 5 because they lie outside the range 4, which has been claimed by the invention.
The sample no. 3.1 only reaches a UE-value which is 8.1%. These 8.1% are smaller than the minimum UE value of 10%. One of the reasons for not reaching the minimum UE value is the carbon content, which at 0.18 wt. % is above the upper limit of 0.12 wt. % set here. Furthermore, the minimum requirement for the fts value of 40% according to formula 3 is not reached.
Although the sample no. 3.2 achieves a sufficiently high UE-value, the fts value at 29% is significantly below ftsmin=40%. From equation (2) an annealing temperature T2 is calculated, which in accordance with the invention for this particular alloy should be at 612.8° C. max. The sample no. 3.2, however, was annealed at relatively high 680° C., which results in an fts-value being too low.
Although the sample no. 3.3 achieved a sufficiently high UE-value, the fts value at 47% is well below the required fts value of 57% pursuant to formula 3. One of the reasons for the failure to reach the minimum fts value lies in the content of manganese which, at 1.83 wt. %, is below the lower limit of 3.5 wt. % set here.
According to the invention, the alloy is thus composed of the following ingredients:
In at least some of the embodiments the carbon content (C) lies in the range 0.003 wt. %≤C≤0.08 wt. %, and/or the manganese content (Mn) in the range 4 wt. %≤Mn≤10 wt. %, in particular in the range 6 wt. %≤Mn≤10 wt. %, since particularly high fts values can be achieved in this case.
In at least some of the embodiments the silicon content (Si) lies in the range 0 wt. %≤Si≤1 wt. %. In particular, the silicon content (Si) is in the range 0.2 wt. %≤Si≤0.9 wt. %.
In at least some of the embodiments the aluminum content (Al) lies in the range 0 wt. %≤Al≤1 wt. %. In particular, the aluminum content (Al) is in the range 0.01 wt. %≤Al≤0.7 wt. %.
In at least some of the embodiments the alloy comprises a sulfur content (S) in wt. %, which is less than 60 ppm.
In at least some of the embodiments the alloy comprises a chromium content (Cr) in the range of 0 wt. % Cr 1 wt. %.
In at least some of the embodiments the alloy comprises one or more than one of the following micro-alloy components:
In at least some of the embodiments the titanium content (Ti), if present, lies in the range 0 wt. %<Ti≤0.12 wt. %.
In at least some of the embodiments the micro-alloy components together have maximum a proportion of 0.15 wt. % of the alloy.
The information made here regarding the composition of the alloy are understood to be in weight percent. The rest of the alloy includes iron (Fe) as well as impurities that cannot be avoided in such a melt. The data in percent by weight always add up to 100 wt. %.
As already described, the method of the invention comprises a special annealing step which is executed after cold rolling step:
Performing an inter-critical box annealing S.2.1 or S.2.2 with a maximum annealing temperature T2 of 684° C.−(517° C.*to the carbon content in wt. %). The carbon content in wt. % is also referred to here as C %. If this intercritical box annealing method is part of a one-step annealing process, then the maximum annealing temperature T2 can even be below these values, as expressed by the formula 648° C.−(352° C.*the carbon content in terms of wt. %).
Exemplary details of a one-step annealing process GR 1 are shown in
The intercritical box annealing, which is also abbreviated to intercritical annealing, is performed with a holding temperature T2 in the α+γ-two-phase region. The area between Ac3 and Ac1. (see
The fully austenitic annealing method S.1 (see
Exemplary details of a two-step annealing process GR 2 are shown in
As can be derived from the different diagrams and the description of these diagrams, it is important for achieving high fts-values, which are above 40%, that the annealing temperature T2 for the intercritical box annealing process is not too high. The maximum annealing temperature T2, which is used for intercritical box annealing processes, is always lower than Ac3 and its upper limit is limited by equations (1) or (2).
The properties of cold strip steel intermediate products of the invention are, inter alia, influenced by the selection of the annealing temperature T1 and/or T2, wherein especially the temperature T2 is dependent on the carbon content in wt. %, and is always less than the maximum annealing temperature Ac3.
Fts-values result for the cold strip steel intermediate products of the invention, which according to equation (3) amount to at least 104*e(−0.001*Rm) at a minimum uniform elongation (Ag) of 10% and a tensile strength (Rm) in the range from 590 MPa to 1350 MPa. These fts values were determined on non-notched flat tensile specimens of the cold strip steel intermediate products.
The cold strip steel intermediate product of the invention is characterized inter alia in that it has a microstructure with the following proportions, if a single-step annealing process GR 1 of
The cold strip steel intermediate product of the invention is characterized inter alia in that it has a microstructure with the following proportions, if a two-step annealing process GR 2 of
This microstructure with a martensite content, a retained austenite content, an alpha-ferrite content and a cementite content provides for the special properties of the cold strip steel intermediate products of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 18183501.8 | Jul 2018 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/067977 | 7/4/2019 | WO | 00 |
