Patent Application
20240141455
The invention relates to a cold-rolled flat steel product, to a method for the production thereof, and to uses of a flat steel product according to the invention.
“Flat steel products” are understood here to be rolled products the length and width of which are each substantially greater than their thickness. These include in particular steel strips, steel sheets, and pre-cut parts obtained therefrom—such as blanks and the like.
In the present text, unless explicitly noted otherwise, information about the alloying constituents is always given in % by mass.
Where formulas or conditions are mentioned in the present text, in which values are calculated or found on the basis of fractions of certain alloying elements, the considered fractions of alloying elements are given in each case in % by mass in these formulae or conditions, unless stated otherwise.
Especially in the field of automobile body construction, there exists a requirement for high-strength steels, which at the same time should have good formability. In particular in the production of components formed into complex shapes, there are high demands with respect to the local shape change capacity and the edge crack resistance, which can be quantified, for example, by good values in a hole expansion test.
A cold-rolled flat steel product is known from EP 2 031 081 B1, which can be hot-dip coated with a zinc-based anticorrosion coating and which has a structure that consists of 20-70% martensite, up to 8% of residual austenite, and has a remainder of ferrite and/or bainite. The flat steel product has a tensile strength of at least 950 MPa and consists of a steel consisting of (in % by weight) C: 0.050-0.105%, Si: 0.10-0.60%, Mn: 2.10-2.80%, Cr: 0.20-0.80%, Ti: 0.02-0.10%, B: <0.0020%, Mo: <0.25%, Al: <0.10%, Cu: up to 0.20%, Ni: up to 0.10%, Ca: up to 0.005%, P: up to 0.2%, S; up to 0.01%, N: up to 0.012%, and iron and unavoidable impurities as the remainder.
Steel concepts of this type are characterized by a low elastic limit ratio which is attributable to significant strength differences between the structural constituents.
In practice, it is found that flat steel products of the type explained above have a particularly high strength, but tend to form edge cracks which impair their processability. Furthermore, such flat steel products require further improved suitability for welding.
Against this background, there is a need to develop a flat steel product which has a steel substrate with high strength, improved weldability, and optimized forming properties, and which can be produced cost-effectively.
Furthermore, a method for producing such a flat steel product, and also uses, should be specified, for which a flat steel product according to the invention is particularly suitable.
A product proposed to address this need has at least the features as described herein.
A method that enables the cost-effective production of a product according to the invention is specified as described herein. Such a product can be provided in the manner indicated as described herein with an anticorrosion coating, based in particular on zinc (“Zn”).
In this case, it should be clear that a person skilled in the art, when implementing the method according to the invention and its variants and expansion possibilities as explained here, will supplement those work steps not explicitly mentioned in the present instance, which they know are regularly applied when performing such methods based on their practical experience.
The steel substrate of a flat steel product according to the invention is accordingly produced from a steel which, in % by mass, consists of C: 0.040-0.100%, Mn: 2.10-2.50%, Si: 0.10-0.40%, Cr: 0.30-0.90%, Ti: 0.020-0.080%, B: 0.0005-0.0020%, N: 0.003-0.010%, Al: up to 0.10%: up to 0.005%, P: up to 0.025%, S; up to 0.010%, Mo: up to 0.20%, Nb: up to 0.050%, Cu: up to 0.10%, V: up to 0.020%, Ni: up to 0.10%, and iron and unavoidable impurities as the remainder.
In this case, the steel substrate of a flat steel product according to the invention has a dual-phase structure which consists of 10-40 vol. % martensite, 30-90 vol. % ferrite, including bainitic ferrite, not more than 5% residual austenite, and of other structural constituents which are unavoidable due to the production process as a remainder, wherein such other structural constituents are present only if the sum of the fractions of the other constituents of the structure is less than 100%.
The fractions of the individual alloying constituents provided according to the invention were determined as follows, wherein the following explanations each also refer to the composition of the steel substrate of a flat steel product according to the invention when only one flat steel product according to the invention is discussed:
A flat steel product according to the invention comprises 0.040-0.100% by mass of carbon (“C”). For C fractions below 0.040% by mass, the strength would decrease too much. The maximum carbon fraction of 0.100% by weight provided according to the invention was selected with regard to good weldability of the steel. Moreover, carbon fractions above 0.100% by weight would lead to the formation of a harder carbon-rich martensite phase, which would significantly increase the difference in hardness between martensite and ferrite. This would have a negative effect on the hole-expansion behavior and the weldability of a flat steel product according to the invention. The positive effects of the presence of C in the steel of a flat steel product according to the invention can be exploited particularly well if the C fraction is at least 0.05% by mass and at most 0.08% by mass.
Silicon (“Si”) is contained in the flat steel product according to the invention in fractions of 0.10-0.40% by mass, in order to increase the strength by the curing effect that Si has on ferrite. The upper limit of the Si fraction is 0.40% by mass, to avoid grain boundary oxidation by which the coatability and the surface properties of the steel could be negatively influenced.
Manganese (“Mn”) in fractions of 2.10-2.50% by mass reliably prevents perlite from forming in the flat steel product according to the invention during cooling in the annealing line. At the same time, Mn in the fractions determined according to the invention mediates the formation of martensite in the structure, and thus contributes substantially to increasing the strength. Among other things, the Mn fractions provided according to the invention compensate for the loss of strength which would otherwise be expected as a result of the C fraction set to comparatively low values in accordance with the invention. The Mn fraction is preferably at least 2.20% by mass and at most 2.40% by mass.
Aluminum (“Al”) in fractions of up to 0.10% by mass is required for deoxidation during steel production.
Calcium (“Ca”) can likewise be added to the steel of a flat steel product according to the invention in fractions of up to 0.005% by mass, in order to deoxidize the steel during steel production. This effect can be achieved by adding at least 0.0005% by mass of Ca.
Chromium (“Cr”) likewise serves to increase the strength in the steel of a flat steel product according to the invention. A Cr fraction of at least 0.30% by mass, in particular at least 0.40% by mass, is required for this purpose. In order to reduce the risk of a pronounced grain boundary oxidation, the upper limit of the range specified according to the invention for the Cr fraction is limited to at most 0.90% by mass, in particular at most 0.80% by mass. In the case of chromium fractions of more than 0.80% by mass, the method for producing the flat steel product obtained according to the invention is to be carried out such that an annealing temperature GT of at least 840° C. is set, in order to obtain the desired dual phase structure and the desired mechanical properties of the flat steel product according to the invention in a reliable manner.
Titanium (“Ti”) is provided in the steel of a flat steel product according to the invention in fractions of 0.020-0.080% by mass, in order likewise to improve the strength by the formation of fine Ti precipitates, such as TiC or Ti(C,N) precipitates, and to obtain a fine-grained structure. In order to achieve this effect particularly reliably, Ti fractions of at least 0.030% by mass can be provided. The quantity of precipitation enabled by the Ti fraction provided according to the invention contributes among other things to the optimal combination of mechanical properties which characterizes a steel according to the invention. The positive influence of the presence of Ti in the steel of a flat steel product according to the invention can be exploited particularly effectively in the case of Ti fractions of up to 0.07% by mass.
The effect of Ti in the steel of a flat steel product according to the invention can additionally be assisted by adding Ti in an amount which corresponds at most to 11 times the respective N and B fractions of the steel of a flat steel product according to the invention. In this embodiment, therefore, the following Ti fraction % Ti applies:
% Ti≤11×(% N+%B)
where % N=the given N fraction and % B=the given B fraction. With such a restriction of the Ti fraction, an optimal amount of Ti precipitates is obtained, and at the same time the formation of boron nitrides is prevented, which would have a negative effect on the formability.
Boron (“B”) is present in the steel of a flat steel product according to the invention in fractions of 0.0005-0.0020% by mass, in order on the one hand to increase the strength, but on the other hand also not to reduce the formability of a flat steel product according to the invention.
The nitrogen (“N”) fraction is limited in the steel of a flat steel product according to the invention to up to 0.010% by mass, so that Ti acts as an alloying element in the structure and is not completely bound with N. Fractions of at least 0.003% by mass N are provided in order to ensure a sufficient amount of Ti(C,N) precipitates in the structure.
In the steel of a flat steel product according to the invention, impurities are permitted which are technically unavoidable in a practical, economic production of a flat steel product according to the invention, but are kept so low that they do not have any adverse effects on the desired properties of a flat steel product according to the invention.
The impurities include fractions of phosphorus (“P”) and sulfur (“S”). The fraction of P is limited to up to 0.025% by mass, in particular less than 0.015% by mass, in order to avoid worsening of the weldability. The S fraction is limited to at most 0.010% by mass in order to avoid the formation of MnS and/or (Mn/Fe)S, which would have a negative effect on the tensile properties of the steel according to the invention.
The total fraction of impurities is limited in the steel of a flat steel product according to the invention to at most 0.5% by mass, wherein an impairment of the properties of the flat steel product at a total of impurities of at most 0.3% by mass is particularly reliably avoided.
Up to 0.20% by mass of molybdenum (“Mo”), up to 0.050% by mass of niobium (“Nb”), up to 0.10% by mass of copper (“Cu”), up to 0.020% by mass of vanadium (“V”) and up to 0.10% by mass of nickel (“Ni”) can optionally be added to the steel according to the invention. The fractions of these elements are limited in such a way that they have only a minor influence on the properties of a flat steel product according to the invention. They can therefore also be “0%” in the technical sense, i.e., be so low that they can be considered impurities, and do not produce any effect in the flat steel product according to the invention.
As a result of the described adjustment according to the invention of the composition of the steel of a flat steel product according to the invention, it is possible to provide a flat steel product consisting of a dual-phase steel, which has a tensile strength Rm of 750-940 MPa, an elastic limit of 440-650 MPa, and an elongation at break A80 of more than 13%, and is characterized by particularly good forming properties with minimized edge crack tendency, and likewise good weldability. The tensile strength Rm, the elastic limit Rp0.2, and the elongation at break A80 are each determined in accordance with DIN ISO 6892 (longitudinal tensile direction; sample form 2).
An essential difference between the invention and, for example, the prior art described at the outset, known from EP 2 031 081 B1, consists in the distribution of the hardness values into the martensitic and ferritic phases, and in the precipitation state of one of the structures of a flat steel product according to the invention, which is characterized by a large amount of fine precipitates. This structure state can be achieved primarily by a carbon fraction limited according to the invention, and a certain addition of Ti and B amounts. In this way, an above-average robust behavior with increasing shape change gradients in the hole expansion test is achieved.
The martensite and ferrite fractions including bainitic ferrite in the structure of a flat steel product according to the invention are quantified by means of image analysis.
As a result of the alloy selection according to the invention, the martensite fraction in the structure of a flat steel product according to the invention is limited to not more than 40 vol. %, wherein at least 10 vol. % of martensite is present in order to secure the required strength.
The rest of the structure of a flat steel product according to the invention, in addition to fractions of not more than 5 vol. % residual austenite, is primarily ferrite, including bainitic ferrite, which may not be more than 90 vol. % and is at least 30 vol. %.
A flat steel product according to the invention displays particularly good forming properties which manifest themselves in high values for the hole expansion ratio HER of greater than 20% (determined according to DIN ISO 16630), and a maximum drawing depth of greater than 33 mm (determined in the limiting dome height (LDH) test with a 100 mm hemispherical die). These are achieved by an early local hardening which is higher than with comparable goods in this strength class, and which are reflected in a tensile strain hardening exponent n, measured in the elasticity interval between 0.2% and 2.2% according to DIN EN ISO 10275:2014, of at least 0.22%.
Due to their particular properties, flat steel products according to the invention are suitable, in particular, for the production of axially loaded components, such as longitudinal members and cross-members, or for producing bending-load-bearing components, such as B-pillars, B-pillar reinforcements, or sills of automobile bodies.
According to the invention, cold-rolled flat steel products obtained according to the invention can be produced by performing at least the following work steps:
wherein the cold-rolled flat steel product heated to the annealing temperature GT is cooled to a cooling end temperature KET in two steps, wherein the cold-rolled flat steel product in the first step of its cooling is cooled from the given annealing temperature GT to an intermediate temperature ZT lying in the range of 750-620° C., with a cooling rate AR1 which is greater than 1.5 K/s, and in the second step from the intermediate temperature ZT to the given cooling end temperature KET, with a cooling rate AR2 for which the following applies: AR2>4×AR1 or
wherein the cold-rolled flat steel product heated to the annealing temperature GT is cooled to a cooling end temperature KET in two steps, wherein the cold-rolled flat steel product in the first step of its cooling is cooled from the given annealing temperature GT to an intermediate temperature ZT lying in the range of 700-450° C., with a cooling rate AR1 which is greater than 5 K/s, and in the second step from the intermediate temperature ZT to the given cooling end temperature KET, with a cooling rate AR2 for which the following applies: AR2<(AR1)/3;
The melting of a melt alloyed according to the invention can likewise take place in a conventional manner, as can the casting of the melt to make the precursor, which is typically a slab or thin slab (work steps a) and b). Slabs in this case typically have thicknesses of 180 mm to 260 mm, while the thicknesses of thin slabs typically lie around 40 mm to 60 mm.
The hot rolling of the precursor can likewise take place in a conventional manner on assemblies known from the prior art. The hot rolling end temperature is set to 850-980° C., preferably to 880-950° C.
After the hot rolling, the hot-rolled strip obtained is cooled to a coiling temperature which is 480-650° C., and is wound into a coil at this temperature. A range of the coiling temperatures which is particularly reliable is limited to at least 500° C. and at most 600° C. At coiling temperatures above 600° C., the risk of grain boundary oxidation increases, which would worsen the surface quality of the flat steel product. At coiling temperatures below 500° C., the strength of the hot-rolled strip decreases greatly, which causes difficulties in the subsequent forming. The coiled hot-rolled flat steel product cools down to room temperature in the coil.
Subsequently, the flat steel product can optionally be descaled. For this purpose, it can, for example, pass through a pickling device in which scale adhering to the flat steel product is removed.
The optionally descaled hot-rolled strip is then cold rolled to form a cold-rolled flat steel product, wherein the total degree of cold rolling KG ([thickness of the flat steel product prior to cold rolling—thickness of the flat steel product after cold rolling]/thickness of the flat steel product prior to cold rolling]×100%) achieved in the course of cold rolling is 25-70%.
If a flat steel product according to the invention is to be coated with an anticorrosion layer based on zinc by hot dip coating, the cold-rolled flat steel product can be produced in accordance with the work steps a)—f), and then can complete the following work steps in a continuous run:
This cooling is carried out in two steps:
According to a first method variant, the cold-rolled flat steel product in the first step of its cooling is cooled from the given annealing temperature GT to an intermediate temperature ZT lying in the range of 750-620° C., with a cooling rate AR1 which is greater than 1.5 K/s, and in the second step from the intermediate temperature ZT to the given cooling end temperature KET, with a cooling rate AR2 for which the following applies: AR2>4×AR1
According to a second method variant, on the other hand, the cold-rolled flat steel product in the first step of its cooling is cooled from the given annealing temperature GT to an intermediate temperature ZT lying in the range of 700-450° C., with a cooling rate AR1 which is greater than 5 K/s, and in the second step from the intermediate temperature ZT to the given cooling end temperature KET, with a cooling rate AR2 for which the following applies: AR2<(AR1)/3; The choice of the respective cooling rates in the first and second steps achieves the desired structure formation of a flat steel product according to the invention.
i) cooling and/or heating the cold-rolled flat steel product from the cooling end temperature KET to a bath entry temperature BT of 450 to 490° C., and conveying the cold-rolled flat steel product through a melt bath consisting of zinc or a zinc alloy, wherein the thickness of the layer formed on the flat steel product is adjusted when exiting the melt bath. The composition of the melt bath can be selected in a conventional manner, wherein the melt bath can be a pure zinc melt, or consists of at least 75% by weight of Zn.
If a cold-rolled flat steel product according to the invention is to remain uncoated or is to be electrolytically coated, an annealing treatment takes place in a continuous furnace at an annealing temperature in the range from 780 to 920° C., with an annealing duration Gt between 10-1000 s. Subsequently, the heated cold-rolled flat steel product is cooled to a cooling end temperature KET in the range 380 to 500° C. in such a way that the cooling of the cold-rolled flat steel product heated to the annealing temperature GT to a cooling end temperature KET occurs in two steps, wherein the cold-rolled flat steel product in the first step of its cooling is cooled from the given annealing temperature GT to an intermediate temperature ZT lying in the range of 700-450° C., with a cooling rate AR1 which is greater than 5 K/s, and in the second step from the intermediate temperature ZT to the given cooling end temperature KET, with a cooling rate AR2 for which the following applies: AR2<(AR1)/3; This is followed by cooling of the cold-rolled flat steel product to room temperature.
Optionally, the obtained cold-rolled flat steel product provided with the anticorrosion coating, or uncoated, can still be subjected to skin-pass rolling in order to optimize its mechanical properties, its surface properties, and its dimensional accuracy. To this end, forming degrees (“skin-pass degrees”) of max. 2%, in particular 0.2-0.7%, have proven successful.
The invention is explained in more detail below with reference to exemplary embodiments.
To test the invention, ten melts A-J were melted, the compositions of which are indicated in Table 1. The melts A-J were cast into slabs in a conventional continuous casting plant, which are subsequently hot-rolled to form hot-rolled strips, coiled to form a coil, and cooled to room temperature. Subsequently, the hot-rolled strips are pickled and cold rolled with a total degree of cold rolling KG, to form a cold-rolled flat steel product present as a cold strip.
In order to coat the cold-rolled flat steel products thus obtained with a Zn-based anticorrosion coating, the cold-rolled flat steel products were annealed at the given annealing temperature GT over a given annealing duration Gt. Starting from the annealing temperature GT, the cold-rolled flat steel products were cooled to a cooling end temperature KET. For this purpose, the cooling of the flat steel product was carried out in one step, or in two steps, wherein the cooling proceeded in the first step of the cooling to an intermediate temperature ZT with a cooling rate AR1, and then in the second step of the cooling to the cooling end temperature KET with a cooling rate AR2 starting from the intermediate temperature ZT (Table 2).
The cooled cold-rolled flat steel products are subsequently heated or cooled to the bath entry temperature BT and conveyed through a melt bath consisting of at least 75% Zn. The thickness of the anticorrosion coatings applied in this way by hot dip coating on the cold-rolled flat steel products was adjusted in a conventional manner by blowing off the excess coating material when the flat steel products exit the melt bath.
After a conventional cooling by means of water or air to room temperature, the cold-rolled flat steel products provided with the anticorrosion coating were subjected to skin-pass rolling, in which they were skin-pass rolled with skin-pass degrees of 0.2-0.7% (skin-pass degree=[(thickness of the flat steel products prior to skin-pass rolling—thickness of the flat steel products after skin-pass rolling)/thickness of the flat steel products prior to skin-pass rolling]×100%).
The tensile strength Rm, the elastic limit Rp0.2 and the elongation A80, and also the hole expansion ratio HER according to DIN ISO 16630 were determined for the cold-rolled flat steel products thus obtained, according to DIN ISO 6892 (longitudinal tensile direction, sample form 2). The structural fractions of ferrite F and martensite M were determined using light microscopy according to DIN 50601: 1985-08. The remaining structure, if present, consisted of small fractions of bainite and residual austenite. The latter was determined by means of standard quantitative phase analysis according to DIN EN 13925 (2003.07) with the aid of Rietveld refinement. These properties are specified in Table 3.
In order to demonstrate the particular effect according to the invention with respect to formability and hole-expansion behavior, the following tests are carried out, beyond the test of hole expansion ratio HER carried out according to DIN ISO 16630:
The steel strips with a tensile strength Rm of at least 750 MPa produced with the alloy concept according to the invention are characterized by the fact that, for a hole expansion test with decreasing cone angle, an above-average increase in the measured hole expansion is achieved when the tests are carried out with cone angles varied in the range of 180° to 50° in order to influence the shape change distribution in a targeted manner in the region of the punched hole, which is close to 0 mm to 5 mm wide.
In these tests, the punched hole is produced by mechanical shear cutting. Identical cutting parameters are set for all samples. The width of the cutting gap is in the range of 9 to 15% of the thickness of the flat steel product being tested. By using punched holes that are punched identically, an influence by the cutting process is excluded, and identical conditions are achieved for all punch geometries.
The material failure is characterized by a constriction or a crack over the entire sheet thickness in the region of the cutting edge. With a diameter of the punched hole of 20 mm, which is significantly larger compared to the test according to DIN ISO 16630, the influence of the sheet thickness in the typical sheet thickness range of 1.0 to 2.0 mm is comparatively low. The achieved hole expansion values of the different punches are more easily compared by a geometric conversion to the center plane of the metal sheet. Assuming “single-axis tensile force” on the edge and using the measured hole expansion, the sheet thickness reduction can be found according to the relationships depicted in Table 4:
The effects which occur in the hole expansion experiments carried out in the manner explained above can be detected by means of FE analysis. The moment of failure and/or the maximum possible expansion is determined by means of video analysis. For this purpose, the process is observed centrally from above by means of a camera. By using a telecentric lens, the hole expansion and/or the diameter of the inner edge delimiting the given hole can be measured before the moment of failure and calculated as a percentage hole expansion with respect to the exit diameter. For this purpose, the image frequency of the video film is at least 10 images per mm of punch path, for a punch speed of 1 mm/s.
In addition, in order to evaluate the global formability in the stretch-forming region, the drawing depth was analyzed in a limiting dome height test (LDH test). In this test, as shown schematically in
C
0.018
E
2.65
0.12
0.030
0.0002
I
0.029
0.45
2.70
0.92
J
0.134
1.86
0.22
C
0.002
E
I
J
| Number | Date | Country | Kind |
|---|---|---|---|
| 21160462.4 | Mar 2021 | EP | regional |
This application is the United States national phase of International Application No. PCT/EP2022/055359 filed Mar. 3, 2022, and claims priority to European Patent Application No. 21160462.4 filed Mar. 3, 2021, the disclosures of which are hereby incorporated by reference in their entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/055359 | 3/3/2022 | WO |
