Patent Application
20190119774
The present disclosure generally relates to flat steel products with optimized strength and elongation characteristics, including methods for producing such flat steel products.
Where reference is made here to flat steel products, this means steel strips, sheets or sheet metal blanks obtained therefrom, such as sheet bars.
Unless explicitly stated otherwise, in the present text and the claims, the contents of particular alloy elements are each reported in % by weight and the proportions of particular microstructure constituents in area.
CA 2 734 976 A1 (WO 2010/029983 A1) discloses a steel having good ductility and formability, which is to have a tensile strength of at least 980 MPa. For this purpose, the steel comprises, as well as iron and unavoidable impurities (in % by weight), 0.17%-0.73% C, up to 3.0% Si, 0.5%-3.0% Mn, up to 0.1% P, up to 0.07% S, up to 3.0% Al and up to 0.010% N. The sum total of the Al and Si contents is to be at least 0.7%. At the same time, in each case in relation to the totality of all microstructure constituents, the martensite content in the steel microstructure is to be 10%-90%, the proportion of residual austenite within the range of 5%-50%, and the proportion of ferritic bainite originating from “upper bainite” at least 5%. “Upper bainite” refers here to a bainite in which fine carbide grains are distributed homogeneously, whereas these are not to be found in “lower bainite”. Higher contents of upper bainite of 17% or more are regarded as advantageous in order to generate the desired high residual austenite contents in the microstructure.
EP 2 524 970 A1 additionally discloses a flat steel product having a tensile strength Rm of at least 1200 MPa and consisting of a steel which, as well as Fe and unavoidable impurities, contains (in % by weight) C: 0.10%-0.50%, Si: 0.1%-2.5%, Mn: 1.0%-3.5%, Al: up to 2.5%, P: up to 0.020%, S: up to 0.003%, N: up to 0.02%, and optionally one or more of the elements “Cr, Mo, V, Ti, Nb, B and Ca” in the following contents: Cr: 0.1%-0.5%, Mo: 0.1%-0.3%, V: 0.01%-0.1%, Ti: 0.001%-0.15%, Nb: 0.02%-0.05%. The sum total Σ(V,Ti,Nb) of the contents V, Ti and Nb here is subject to the following criterion: Σ(V,Ti,Nb)≤0.2%, B: 0.0005%-0.005%, Ca: up to 0.01%. At the same time, the flat steel product has a microstructure having (in area %) less than 5% ferrite, less than 10% bainite, 5%-70% unannealed martensite, 5%-30% residual austenite and 25%-80% annealed martensite, with at least 99% of the iron carbides present in the annealed martensite having a size of less than 500 nm. Owing to its minimized proportion of overannealed martensite, a flat steel product having such characteristics has optimized formability.
EP 2 524 970 A1 likewise discloses a process for producing a flat steel product of the type elucidated above. In this process, first of all, a flat steel product having the aforementioned composition is heated at a heating rate θH1, θH2 of at least 3° C./s to an austenitization temperature THZ above the A3 temperature of the steel of the flat steel product and of not more than 960° C. The flat steel product is kept at that temperature for an austenitization period tHZ of 20-180 s, in order then to be cooled to a cooling finish temperature. The latter is greater than the martensite finish temperature and less than the martensite start temperature, the cooling being effected at a cooling rate at least equal to a minimum cooling rate determined as a function of the alloy contents of the steel. Then the flat steel product is kept at the cooling finish temperature for 10-60 s, in order then to be heated at a heating rate of 2-80° C./s to a partitioning temperature of 400-500° C. This may be followed by an isothermal hold of the flat steel product at the partitioning temperature over up to 500 s. Subsequently, the flat steel product is cooled down at a cooling rate of 3-25° C./s.
In the known process elucidated above, the heating and the optional additional holding at the partitioning temperature result in enrichment of the residual austenite in the microstructure of the flat steel product with carbon from the oversaturated martensite. This operation also is referred to in the art as “partitioning of the carbon” or “partitioning”. The partitioning can be conducted as early as during the heating, as what is called “ramped partitioning”, by means of holding at the partitioning temperature after the heating (called “isothermal partitioning”), or by means of a combination of isothermal and ramped partitioning. The slower heating rate which is the aim in ramped partitioning as compared with isothermal partitioning permits particularly exact actuation of the partitioning temperature specified in each case with a reduced energy input.
The steels having the characteristics and having been processed as elucidated above are among what are called the “AHSS steels” (advanced high strength steels).
Modern variants of these steels and flat steel products produced therefrom have very high strength with simultaneously high elongation, and are therefore particularly suitable for the production of safety-relevant components of automobile bodywork which are to absorb deformation energy in the event of a crash. However, it is found in practice that high residual austenite contents in the microstructure of such steels can improve the uniaxial elongation thereof by virtue of the known TRIP effect, but that they are not reliably successful in achieving equally good formability in all directions, as indicated, for example, by good hole expanding characteristics.
Although certain example methods and apparatus have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents. Moreover, those having ordinary skill in the art will understand that reciting ‘a’ element or ‘an’ element in the appended claims does not restrict those claims to articles, apparatuses, systems, methods, or the like having only one of that element, even where other elements in the same claim or different claims are preceded by “at least one” or similar language. Similarly, it should be understood that the steps of any method claims need not necessarily be performed in the order in which they are recited, unless so required by the context of the claims. In addition, all references to one skilled in the art shall be understood to refer to one having ordinary skill in the art.
One example object of the present disclosure is to provide a flat steel product that has not just an optimized combination of high strength and elongation, but also, coupled with improved use properties such as good suitability for welding, surface characteristics and suitability for coating with a metallic protective coating, has a microstructure that assures optimized formability irrespective of the direction of forming.
A process for producing such a flat steel product was likewise to be specified.
Advantageous configurations of the invention are specified in the dependent claims and are elucidated in detail hereinafter, as is the general concept of the invention.
A flat steel product of the invention accordingly features a tensile strength Rm of at least 950 MPa, a yield point of at least 800 MPa and an elongation at break A50 determined according to DIN EN ISO 6892, sample shape 1, of at least 8%. A flat steel product of the invention consists here of a steel consisting of, as well as iron and unavoidable impurities, (in % by weight)
ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si)
1.5≤ψ≤3
The invention is based on the finding that, through the choice of a suitable alloy, it is possible to obtain a flat steel product in which a microstructure comprising minimum residual austenite contents at most and characterized by a high content of annealed martensite and by ultrafinely distributed unannealed martensite results in a high strength coupled with very good deformability.
Typical tensile strengths Rm of flat steel products of the invention are 950-1300 MPa, coupled with a yield point which is at least 800 MPa and can reach as far as the respective tensile strength. The elongation A50 of flat steel products of the invention is typically 8%-20%. At the same time, a flat steel product of the invention, in the hole expanding test according to ISO 16630, regularly achieves hole expansion ratios of at least 30%.
These combinations of properties are accomplished in accordance with the invention through the exactly judged addition of inexpensive alloy constituents. These are matched to one another such that the desired mechanical properties are reliably achieved and the flat steel product obtained simultaneously exhibits good weld- and coatability.
Of essential significance here is the establishment of a suitable ratio between the elements that affect austenite formation and the hardenability of the steel, and the elements that suppress carbide formation. This ratio in the case of an alloy according to the invention is adjusted via the factor ψ, which is affected by the respective C, Mn, Cr, Al and Si contents of the steel. The factor ψ is not to be less than 1.5. Excessively high contents of silicon or aluminum would have an adverse effect on the coatability (silicon) or the castability (aluminum) of the steel. In the case of inadequate contents of carbon, manganese or chromium, the required strength would not be achieved. Relatively high values for the factor ψ of at least 1.6 have been found to be advantageous for the establishment of a stable production process, and values for the factor ψ of at least 1.8 have been found to be particularly advantageous for production stability. Excessive carbon and manganese can lead to an elevated residual austenite content, which would in turn result in lower formability. This is avoided in that the upper limit for the range in which the ψ factor of a steel of the invention lies has been set to the value of 3.0.
Carbon has several important functions in the steel of the invention. Firstly, the C content plays a major role in the formation of austenite and adjustment of the A3 temperature. An adequate C content enables full austenitization even at temperatures of less than 930° C. In the subsequent quenching, the residual austenite is stabilized by carbon. This stabilization can be assisted by an additional heat treatment step as envisaged by the invention in the process of the invention. The strength of the martensite is also greatly affected by the C content of the steel. On the other hand, the martensite start temperature is shifted to ever lower temperatures with rising C content, which leads to challenges in the production. For these reasons, the invention envisages, in the steel of a flat steel product of the invention, a C content of 0.05%-0.2% by weight, especially at least 0.065% by weight of C, and in practice the positive effect of C in the steel of the invention can be exploited in a particularly reliable manner when the C content is 0.07%-0.19% by weight.
For the specific judgement of the particular C content in each case, within the limits envisaged in accordance with the invention, it is also possible to cite what is called the carbon equivalent “CE”, the value of which is influenced to a crucial degree by the C content. For calculation of the carbon equivalent CE, the American Welding Society has proposed the following formula:
CE=% C+(% Si+% Mn)/5+(% Cr+% Mo)/6
According to the invention, the carbon equivalent CE should be not more than 1.1% by weight, in order to assure good weldability. Particularly good suitability for welding can be assured in that the CE value is limited to not more than 1.0% by weight. However, the CE value should not be less than 0.254% by weight and especially not less than 0.29% by weight, in order to obtain the effect of the alloy elements that affect the calculation of the carbon equivalent CE and are envisaged in accordance with the invention.
The presence of silicon in the steel of a flat steel product of the invention suppresses the formation of cementite, which would bind carbon that would then no longer be available for the stabilization of the residual austenite, and which would worsen the elongation. The same effect can also be achieved by including Al in the alloy. However, a minimum of 0.2% by weight of Si should be present in the steel envisaged in accordance with the invention. However, Si contents of more than 1.5% by weight would have an adverse effect on the surface quality of a flat steel product of the invention. Therefore, in a flat steel product of the invention, the Si content is 0.2%-1.5% by weight, and in practice Si contents of at least 0.25% by weight or at most 0.95% by weight have been found to be particularly favorable and those of at most 0.63% by weight to be very particularly favorable.
Aluminum is added to the steel of a flat steel product of the invention in steel production for deoxidation and for binding of any nitrogen present. Al can additionally also be used for the suppression of cementite. However, in the presence of higher contents of Al, there is also a rise in the austenitization temperature. Therefore, the Al content of a steel envisaged for a flat steel product of the invention is limited to 0.01%-1.5% by weight. If low austenitization temperatures are to be assured, it may be appropriate to limit the Al content to a maximum of 0.44% by weight, especially to 0.1% by weight. Moreover, higher Al contents have an adverse effect on castability in steel production. Al contents of not more than 1.0% by weight, especially not more than 0.44% by weight, have been found to be favorable for assuring particularly good castability. In addition, aluminum can be bound by nitrogen to give aluminum nitride. Aluminum nitride precipitates present in the flat steel product can have an unfavorable effect on the formability of the flat steel product. Thus, with regard to optimization of formability, it may be appropriate to limit the Al content to not more than 1.0% by weight, especially to not more than 0.44% by weight.
In order to rule out any adverse effect of Si and Al in the flat steel product of the invention, the sum total of the contents of Al and Si in the steel of a flat steel product of the invention can be limited to not more than 1.7% by weight, and particularly favorable upper limits here have been found to be not more than 1.5% by weight, especially not more than 1.0% by weight, particularly with regard to optimization of suitability for welding. With regard to optimization of formability, advantageous upper limits for the sum total of the contents of Al and Si have likewise been found to be not more than 1.0% by weight, especially not more than 0.4% by weight.
Manganese is important for the hardenability of the steel of a flat steel product of the invention and additionally prevents the formation of unwanted pearlite during the cooling. The presence of Mn thus enables the formation of a starting microstructure (martensite and residual austenite) suitable for the formation of the microstructure stipulated in accordance with the invention. However, too high a Mn concentration would have an adverse effect on the elongation and weldability of the steel. Therefore, the range envisaged for the Mn content in accordance with the invention is 1.0%-3.0% by weight, especially at least 1.5% by weight or at most 2.4% by weight.
Phosphorus has an adverse effect on the weldability of a flat steel product of the invention. The P content should be as low as possible, but at least should not exceed 0.02% by weight, and should especially be less than 0.02% by weight or less than 0.018% by weight.
The presence of effective contents of sulfur in the steel of a flat steel product of the invention would lead to formation of sulfides, especially MnS or (Mn,Fe)S, which would have an adverse effect on the elongation. In order to avoid this, the S content of the steel should be kept as low as possible, but at least should not be higher than 0.005% by weight, especially less than 0.005% by weight or less than 0.003% by weight.
In order to avoid the formation of nitrides which could be detrimental to formability, the N content of the steel of a flat steel product of the invention is limited to not more than 0.008% by weight. Advantageously, the N content, for avoidance of any adverse effect, should be below 0.008% by weight, especially less than 0.006% by weight.
Chromium in contents of up to 1.0% by weight can optionally be utilized in the steel envisaged in accordance with the invention as an effective inhibitor of pearlite, and additionally contributes to strength. In the case of contents of more than 1.0% by weight of Cr, there is the risk of marked grain boundary oxidation. In order to be able to utilize the positive effect of Cr, at least 0.05% by weight is required. The presence of Cr has a particularly favorable effect in the steel of a flat steel product of the invention when at least 0.15% by weight of Cr is present, and an optimal effect is achieved at contents of up to 0.8% by weight.
Optionally, the steel of a flat steel product of the invention may additionally also contain molybdenum in contents of 0.05%-0.2% by weight. Mo in these contents likewise particularly effectively suppresses the formation of unwanted pearlite.
The steel of a flat steel product of the invention may additionally optionally contain contents of one or more micro alloy elements, in order to promote strength through the formation of very finely divided carbides. It has been found that contents of Ti and Nb are particularly suitable for this purpose.
Ti contents of at least 0.005% by weight and Nb contents of at least 0.001% by weight each lead, alone or in combination with one another, to freezing of the particle and phase boundaries during the heat treatment that a flat steel product of the invention undergoes in the course of production thereof in accordance with the invention. Ti can additionally be utilized for binding of the nitrogen present in the steel, in order to enable an effect of other alloy elements, especially boron. It has been found that particularly advantageous Ti contents are those of at least 0.02% by weight. However, too high a concentration of micro alloy elements would lead to carbides of excessive dimensions, which could initiate cracks at high degrees of deformation. Therefore, the Ti content of the steel of a flat steel product of the invention is limited to not more than 0.2% by weight and the Nb content thereof to not more than 0.05% by weight, and it is found to be advantageous for avoidance of adverse effects of the presence of micro alloy elements when the sum of the contents of Nb and Ti does not exceed 0.2% by weight.
The boron likewise optionally present in the steel of a flat steel product of the invention segregates to the phase boundaries and attenuates their movement. This leads to a fine-grain microstructure, which has an advantageous effect on the mechanical properties. In order that the effect of B can be utilized, Ti can be included in the steel alloy, as mentioned above. In order to be able to utilize the positive effect of B, the steel envisaged in accordance with the invention must contain at least 0.0001% by weight of B. In the case of contents of more than 0.005% by weight, no further increase in the positive effect of B can be identified.
In order to protect it from corrosive attacks, the flat steel product of the invention may have been provided with a metallic protective coating. This may especially have been applied by melt dip coating. Suitable coatings here for a flat steel product of the invention are especially Zn-based coatings.
The process of the invention for producing a high-strength flat steel product comprises the following operating steps:
ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si)
1.5≤ψ≤3;
(TMS−175° C.)<TQ<TMS
The principle of the procedure of the invention is illustrated in the diagram appended as
In operating step a), a flat steel product consisting of a steel having the above-elucidated composition is provided. The flat steel product provided may especially be a cold-rolled flat steel product. However, it is also conceivable to process a hot-rolled flat steel product in the inventive manner.
For the heating of the flat steel product to the austenitization temperature THZ (operating step b)), two steps with one following on from the other without interruption are possible in principle, in which case the flat steel product in the first step is heated at a heating rate ΘH1 of 5-25 K/s up to an inflection temperature TW of 200-400° C. Favorable values of ΘH1 for the productivity of the process have been found to be at least 5 K/s, while a heating rate ΘH1 of more than 25 K/s has been found to be very energy-intensive and costly. Subsequently, the heating in the second step is continued at a heating rate ΘH2 of 2-10 K/s until the austenitization temperature THZ has been attained. In the second heating step, the alloy elements present in the flat steel product can diffuse within the flat steel product during heating operation. As the heating rate increases, there is a decrease in the time available for the diffusion process and hence for the homogenization of the alloy element distribution of the flat steel product. Inhomogeneously distributed alloy elements can lead to locally different microstructure transformations. For establishment of a homogeneous microstructure, it has been found to be advantageous to limit the heating rate ΘH2 to a maximum of 10 K/s. At the same time, values for the heating rate ΘH2 of less than 2 K/s have been found to be unfavorable for the economic viability of the process. Since there is an overlap in the ranges mentioned for the heating rates ΘH1, ΘH2, the heating to the austenitization temperature can also be effected in one run with a constant heating rate of 5-10 K/s. In that case, the heating rates θH1 and θH2 in operating step b) are the same.
The austenitization temperature THZ must be above the A3 temperature. The A3 temperature is dependent on the analysis and can be estimated by the following empirical equation (alloy contents used in % by weight):
A3[° C.]=910−203√{square root over (% C)}−15.2% Ni+44.7% Si+31.5% Mo−21.1% Mn
The alloying of the steel selected in accordance with the invention permits restriction of the austenitization temperature THZ to a maximum of 950° C. and hence allows the operating costs incurred for the performance of the process of the invention to be limited.
In order to prevent large austenite grains from forming, which would have an adverse effect on formability, the austenitization period tHZ over which the flat steel product is kept at the austenitization temperature THZ in operating step c) is limited to 5-15 seconds, where the austenitization period tHZ may be less than 15 s in order to avoid any unwanted grain growth.
In operating step d), there follows controlled and gradual cooling of the flat steel product proceeding from the austenitization period tHZ. This cooling can extend over 50-300 seconds and has to end at an intermediate temperature TK no lower than 680° C., in order to avoid the unwanted formation of ferrite. The upper limit in the intermediate temperature TK is preferably at temperatures of not more than A3, and is typically restricted to 775° C., since, in the case of higher intermediate temperatures TK, the cooling output required for the subsequent cooling is disproportionately high and thus puts the economic viability of the process into question.
After the gradual cooling in operating step d), the flat steel product, in operating step e), is quenched to an analysis-dependent cooling finish temperature TQ at a high cooling rate θQ. The high cooling rate θQ can be achieved, for example, with modern gas jet cooling.
The minimum cooling rate θQ necessary to avoid ferritic and bainitic transformation is more than 30 K/s. There is typically an upper limit to the cooling rate θQ arising from the plant, which is typically not more than 200 K/s. The range within which the cooling finish temperature TQ lies is limited at the upper end by the martensite start temperature TMS, and at the lower end by a temperature which is 175° C. below the martensite start temperature TMS ((TMS−175° C.)<TQ<TMS).
The martensite start temperature can be estimated by means of the following equation (alloy contents used in % by weight):
T
MS(° C.)=539° C.+(−423% C−30.4% Mn−7.5% Si+30% Al)° C./% by wt.
In operating step f), the flat steel product is kept at the cooling finish temperature TQ for a holding period tQ of 10-60 seconds, in order to establish the microstructure. In the course of this step, a martensitic microstructure is obtained with up to 30% residual austenite. The amount of martensite produced in this step depends essentially on the degree to which the cooling finish temperature is below the martensite start temperature TMS. The holding period tQ is at least 10 seconds, in order to assure homogenization of the temperature in the flat steel product and hence a homogeneous microstructure. In the case of longer holding periods of more than 60 seconds, the homogenization of the temperature is complete. The holding period tQ is not more than 60 seconds, in order to increase the productivity of the process.
By contrast with the prior art described at the outset, it is not an aim of the invention to stabilize residual austenite down to room temperature. Instead, the heat treatment of the flat steel product conducted in operating step g) has the aim of controlled redistribution of the carbon such that the microstructure of the flat steel product obtained on conclusion of the process consists essentially of two different kinds of martensite, namely an annealed martensite and an unannealed martensite.
According to the invention, operating step g) comprises two process variants g.1) and g.2), of which the first variant g.1) leads to an uncoated flat steel product of the invention and the second variant g.2) to a flat steel product of the invention provided with a Zn coating.
The temperature regime in each of the variants g.1), g.2) of the operating step g) is chosen such that the existing residual austenite present in the microstructure is enriched with carbon from the oversaturated martensite. The formation of carbides and the breakdown of residual austenite is deliberately suppressed via the inventive limitation of the total treatment period tBT. This period is 10-1000 seconds in order to enable sufficient redistribution of the carbon.
In respect of the first process variant g.1), the treatment of the flat steel product in operating step g) comprises keeping the flat steel product over the entire treatment period tBT at a treatment temperature TB at least equal to the cooling finish temperature TQ and not higher than 550° C., and a cooling finish temperature TQ of not more than 500° C. has been found to be particularly favorable. In the case of variant g.1), the treatment temperature TB may also be higher than the cooling finish temperature TQ. In this case, the flat steel product, proceeding from the cooling finish temperature TQ, is heated to the respective treatment temperature TB, where the heating should be effected at a heating rate ΘB1 of less than 80 K/s.
In the second alternative of operating step g), by contrast, the flat steel product is brought to a treatment temperature TB of 400-500° C. at a heating rate ΘB1 of less than 80 K/s, in order to enrich the residual austenite with carbon from the oversaturated martensite. The formation of carbides and the breakdown of residual austenite are deliberately suppressed by the inventive limitation of the total treatment period tBT, which in this variant g.2) of operating step g) is composed of the heating time tBR required for the heating and the holding period tBI over which the flat steel product is kept under isothermal conditions at the temperature TB. Given a sufficiently gradual heating rate ΘB1, the isothermal hold can also be dispensed with, and so the holding period tBI can be “0”.
In the second variant g.2) of operating step g), the flat steel product, after the heating and the optional hold at the treatment temperature TB, undergoes a melt dip-coating operation in which it is coated with a Zn coating. For this purpose, the treatment temperature TB can be chosen such that it corresponds to the inlet temperature at which the flat steel product is to enter the respective melt bath. Typically, for this purpose, the treatment temperatures TB are in the range of 450-500° C. This melt bath typically comprises, as well as zinc and unavoidable impurities, a total of up to 3.0% by weight of one or more elements from the group consisting of Al, Mg, Si, Pb, Ti, Ni, Cu, B and Mn.
Irrespective of which variant has been chosen, the flat steel product, on conclusion of operating step g), for new production of martensite, is cooled in a controlled manner at a cooling rate θB2 of more than 5 K/s, the cooling rates typically being not more than 50 K/s. θB2 is more than 5 K/s, in order to avoid the formation of pearlite and ferrite.
The process of the invention can be conducted in a continuous run in conventional calcining systems or belt coating systems that are typically provided for the purpose.
The flat steel product of the invention has a microstructure consisting
The microstructure of a flat steel product of the invention, with a mean grain size of less than 2 μm, is very fine and can barely be assessed by means of standard light-optical microscopy. Therefore, an assessment by means of scanning electron microscopy (SEM) with a minimum of 5000-fold magnification is recommended.
The maximum permissible residual austenite content, even in the case of high magnification, can be determined only with difficulty by light microscopy or scanning electron microscopy. Therefore, a quantitative determination of the residual austenite by means of x-ray diffraction (XRD) is recommended (according to ASTM E975), by which the residual austenite content is reported in % by volume.
Another measure that can be employed for the quality of the mechanical properties of a flat steel product of the invention is the distortion of the crystal lattice. This lattice distortion is very important for the initial resistance to plastic deformation. A suitable method for the measurement and quantification of lattice distortion is electron backscatter diffraction (EBSD). By the EBSD method, a sample is scanned point by point by SEM, with recording of a diffraction pattern at every measurement point, from which it is possible to determine the crystallographic orientation. Details of the measurement and of the various evaluation methods can be read in the handbooks. A useful EBSD evaluation method is what is called the kernel average misorientation (KAM—further description in the handbook “OIM Analysis v5.31” from EDAX Inc., 91 McKee Drive, Mahwah, N.J. 07430, USA), wherein the orientation of a measurement point is compared with the neighboring points. Beneath a threshold value, typically 50, adjacent points form part of the same (deformed) grain. Above the threshold value, the adjacent points form part of different (sub-)grains. Because the microstructure is so fine, a maximum step width of 100 nm is recommended in EBSD. For the assessment of the microstructure of the flat steel products of the invention, the KAM of the third adjacent points is evaluated. A flat steel product of the invention must have a mean KAM value from a measurement region of at least 75 μm×75 μm of more than 1.200, preferably more than 1.250
The invention is elucidated in detail hereinafter by working examples.
To test the invention, samples of steel sheets produced in a conventional manner have been provided, which consisted of steels A-I with the compositions specified in table 1.
Table 1 additional states, for each of the steels A-I, the factor ψ and the carbon equivalent CE which have been calculated by the already above-elucidated formulae
ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si)
and
CE=% C+(% Si+% Mn)/5+(% Cr+% Mo)/6
where % C is the respective C content, % Si the respective Si content, % Mn the respective Mn content, % Cr the respective Cr content, % Mo the respective Mo content and % Al the respective Al content of the steels A-I.
Steels E, F and G accordingly did not meet the demands on the tuning of the alloy elements essential to austenite formation and hardenability that are stipulated in accordance with the invention by the factor ψ.
Samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60 manufactured from steels A-I have undergone the process sequence shown in
The aforementioned parameters employed in the experiments are specified in table 2. Of the samples 1-7, 11, 12, 16-23, 28-31 and 44-55 consisting of the inventive steels A-D, H and I, accordingly, samples 3 (θQ<30 K/s), 11 (THZ<A3), 18 (TQ>500° C.), 19 (θQ<30 K/s), 28 (THZ<A3), 29 (tHZ>15 s) and 48 (θB2<5 K/s) have not been treated in accordance with the invention.
In the context of the last cooling, samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60, in the cases where the treatment temperature TB was at a level of about 450° C. sufficient for entry into a Zn melt bath, could have passed through a melt bath. In the context of the experiments, however, this has been dispensed with, and so it did not affect the results of the study.
The mechanical properties of yield point Rp0.2, tensile strength Rm, the Rp0.2/Rm ratio, elongation at break A50 (according to DIN EN ISO 6892, sample form 1), the product Rm*A50, and the hole expansion ratios λ1, λ2 (according to ISO 16630) have been determined on the samples obtained after the heat treatment. Likewise ascertained have been the microstructure proportions of ferrite “F”, annealed martensite “AM”, residual austenite “RA”, unannealed martensite “M” and bainite “B”, and also the value “KAM” determined in accordance with the kernel average misorientation. The property values in question are reported in table 3 for each of the samples.
The mechanical properties attained in the calcined material with a quantification of the microstructure can be found in table 3. In the case of the samples that fulfill both the specifications of the invention in relation to the alloy of the respective steel and the conditions of the invention for the heat treatment, it is regularly the case that yield points Rp0.2 of more than 800 MPa, tensile strengths Rm of more than 950 MPa, and elongation at break values A50 of more than 8% are achieved, combined with hole expansion ratios λ1, λ2 of regularly more than 30%.
Comparative examples B11 and D28, by contrast, illustrate the effect of an insufficient austenitization temperature THZ. In these examples, the microstructure has not been fully austenitized, and so too much ferrite forms in the microstructure. This leads to extremely localized damage and early failure during forming.
Comparative example D29 shows how austenitization for too long a period at high temperatures can adversely affect formability.
Comparative examples A3 and C19 show that, in the case of excessively low cooling rates θQ, the desired yield point is not attained, which is attributable to the fact that ferrite formation could not be adequately prevented.
Comparative example C18, which was produced with too high a cooling finish temperature TQ, shows a yield point below that desired and low hole expansion ratios. These are attributable to an elevated level of ferrite and bainite in the microstructure.
Comparative examples E33-E35 and E56-E58 show a yield point and strength below those desired, which is attributable to the composition not in accordance with the invention and too high a ferrite content in the microstructure obtained. The high ferrite content is caused by inadequate prevention of carbide formation as a result of too low a silicon content and too low a content of aluminum and silicon in relation to carbon, manganese and chromium, and hence too high a ψ factor.
Finally, comparative examples F39, F40, F59 and F60 show the effects of too low a ψ factor, which also leads to departures from the microstructure desired. The minimum strength was attained in some cases, but the yield point and the hole expansion here are not within the target range.
Comparative example G43 makes it clear that too high a factor leads to excessively high residual austenite contents and reduced formability, which is manifested in poor hole expansion values λ1, λ2.
Comparative example 148 illustrates that too low a cooling rate θB2 leads to increased ferrite formation and hence to low yield points.
0.10
3.46
NO
0.8
NO
3.53
NO
YES
YES
−16
780
−21
510
−15
775
25
−21
−22
−20
−21
−22
−22
661
28
10
1.19
NO
565
26
10
1.03
NO
630
15
18
15
55
30
1.01
NO
695
29
20
1.09
NO
701
24
17
20
30
40
8
1.03
NO
29
4
NO
416
30
15
45
9
1.22
NO
277
45
10
40
1.03
NO
283
40
10
45
0.98
NO
428
20
23
35
30
30
1.02
NO
442
17
16
35
30
34
1.05
NO
26
23
10
40
5
40
5
1.11
NO
690
10
8
429
25
25
45
1.17
NO
398
30
15
55
1.13
NO
521
15
35
50
NO
491
26
6
28
NO
405
21
20
15
35
40
10
1.18
NO
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/059968 | May 2015 | EP | regional |
This application is a U.S. National Stage Entry of International Patent Application Serial Number PCT/EP2016/059960, filed May 4, 2016, which claims priority to International Patent Application Serial Number PCT/EP2015/059968 filed May 6, 2015, the entire contents of both of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/059960 | 5/4/2016 | WO | 00 |
