Abstract
A high-strength multi-phase steel having tensile strengths of no less than 580 MPa, preferably with a dual-phase structure for a cold-rolled or hot-rolled steel strip having improved forming properties, in particular for lightweight vehicle construction is disclosed, containing the following elements (contents in % by mass): C 0.075 to 0.105; Si 0.200 to 0.300; Mn 1.000 to 2.000; Cr 0.280 to 0.480; Al 0.10 to 0.060; P 0.020; Nb 0.005 to 0.025; N 0.0100; S 0.0050; the remainder iron, including conventional steel-accompanying elements not mentioned above.
Claims
1. A method for producing a cold or hot rolled steep strip, said method comprising: providing a high-strength multiphase steel with minimal tensile strengths of 580 MPa for a cold or hot rolled steel strip composed of the following elements in weight %: C 0.075 to 0.105 Si 0.200 to 0.300 Mn 1.000 to 2.000 Cr 0.280 to 0.480 Al 0.010 to 0.060 P 0.020 Nb 0.005 to 0.025 N 0.0100 S 0.0050 remainder iron including usual steel accompanying elements not mentioned above, wherein boron is limited to unavoidable steel accompanying element amounts in the high-strength multiphase steel; producing a cold rolled or hot rolled strip from the high-strength multiphase steel; adjusting the Mn content of the high-strength multiphase steel as a function of a different thickness of the strip to obtain comparable material properties of the strip at different thicknesses of the strip, wherein the Mn-content is adjusted to 1.000 to 1.500% at strip thicknesses 0.50-1.00 mm, to 1.250 to 1.750% at strip thicknesses 1.00-2.00 mm and to 1.500 to 2.000% at strip thicknesses 2.00-4.00 mm; heating the cold or hot rolled steel strip in a continuous annealing furnace to an annealing temperature in the range of about 700 to 950 C. to produce an annealed strip; cooling the annealed strip from the annealing temperature to a first intermediate temperature of about 300 to 500 C. at a cooling rate between about 15 and 100 C./s; and cooling the strip to room temperature.
2. The method of claim 1, wherein the Nb content is 0.005 to 0.020%.
3. The method of claim 1, wherein the N content is 0.0090%.
4. The method of claim 1, wherein the N content is 0.0080%.
5. The method of claim 1, wherein for reaching a minimal tensile strength of 780 MPa the Mn content of the steel is 1.500 to 5 2.000 and the steel strip is heated in the heating step below the transformation point A.sub.c1 but not below 700 C.
6. The method of claim 1, wherein for reaching a minimal tensile strength of 780 MPa the Mn content of the steel is 1.500 to 5 2.000, the steel strip has roll reduction degrees of greater than 75% and is heated in the heating step between A.sub.c1 and A.sub.c3.
7. The method of claim 1, further comprising producing in a system multiple of said strip, said multiple strips having different thicknesses and adjusting a throughput speed of the system as a function of the different thicknesses of the strips during heat treatment so as to adjust comparable microstructure states and mechanical characteristic values among the multiple strips.
8. The method of claim 1, further comprising skin passing the steel strip subsequent to the heating and cooling steps.
9. The method of claim 1, further comprising stretch leveling the steel strip subsequent to the heating and cooling steps.
10. The method of claim 1, having a dual-phase microstructure.
11. The method of claim 1, wherein the strip is cooled from the first intermediate temperature to a second intermediate temperature of about 200 to 250 C. with a cooling rate between about 15 and 100 C./s, and subsequently cooled on air with a cooling rate of about 2 to 30 C./s until reaching the room temperature.
12. The method of claim 11, further comprising hot dip coating the strip in a hot dip bath, wherein subsequent to the healing and subsequent cooling the cooling is hated prior to entering into the hot dip bath, and after the hot dip coating the cooling is continued with a cooling rate between about 15 and 100 C./s until reaching the second intermediate temperature of about 200 to 250 C., and subsequently the steel strip is cooled on air with a cooling rate between about 2 and 30 C./s until reaching the room temperature.
13. The method of claim 11, further comprising hot dip coating the steel strip in a hot dip bath, wherein after the heating and subsequent cooling to the second intermediate temperature of about 200 to 250 C. and prior to entering the hot dip bath the temperature is held for about 1 to 20 s and subsequently the steel strip is reheated to the temperature of about 420 to 470 C. and after the hot dip coating the steel strip is cooled until reaching the second intermediate temperature of about 200 to 250 C. with a cooling rate between about 15 and 100 C./s, and subsequently the steel strip is cooled on air with a cooling rate of about 2 and 30 C./s until reaching the room temperature.
14. The method of claim 1, wherein the strip is cooled by maintaining a cooling rate between about 15 to 100 C./s from the first intermediate temperature to the room temperature.
Description
BRIEF DESCRIPTION OF THE DRAWING
(1) Further features, advantages and details of the invention will become apparent from the following description of exemplary embodiments shown in the drawing.
(2) It is shown in:
(3) FIG. 1: schematically the process chain for the production of the steel according to the invention
(4) FIG. 2: results of a hole expansion test (sheet thickness 2.50 mm) exemplary for the steel according to the invention (variant 1) relative to the state of the art
(5) FIG. 3: examples for analytical differences of the steel according to the invention relative to the standard grade, which exemplifies the state of the art
(6) FIG. 4a: Examples for mechanical characteristic values (transversely and longitudinally to the rolling direction) of the steel according to the invention compared to the standard grade which exemplifies the state of the art in the strength class HCT600X.
(7) FIG. 4b: regression calculations for mechanical characteristic values transversely to the rolling direction of the steel according to the invention variant 1, 2 and 3
(8) FIG. 4c: example for mechanical characteristics (transversely to the rolling direction) of the steel according to the invention (variant 1) compared to the standard grade which exemplifies the state of the art in the strength class HCT780X for sheet thickness <1 mm.
(9) FIG. 4d: example for mechanical characteristic values (transversely to the rolling direction) of the steel according to the invention variant 1 in the strength class HDT580X for strip thickness 2.50 mm.
(10) FIG. 5: schematically the time temperature course of the process steps hot rolling and continuous annealing, exemplary for variant 1
(11) FIG. 6: schematic ZTU diagram for the steel according to the invention with the variants 1, 2 and 23
(12) FIG. 7: mechanical characteristic values (longitudinally to the rolling direction) when varying the rolling degrees (?) (exemplary variant 1)
(13) FIG. 8: overview over the strength classes that can be set with the alloy concept according to the invention (exemplary for variant 13)
(14) FIG. 9a: temperature-time curve (schematic, method 1)
(15) FIG. 9b: temperature-time curve (schematic method 2)
(16) FIG. 9c: temperature-time curve (schematic, method 3)
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
(17) FIG. 1 shows schematically the process chain for producing the steel according to the invention. Shown are the different process routes with regard to the invention. Up to position 5 (pickling) the process route is the same for all steels according to the invention, thereafter divergent process routes follow depending on the desired results. For example the pickled hot strip can be galvanized or cold rolled and galvanized. Or it can be soft annealed, cold rolled and galvanized.
(18) FIG. 2 shows results of a hole expansion test (relative values compared to each other). Shown are the results of the hole expansion test for a steel according to the invention (variant 1, see FIG. 3) compared to the standard grades, as reference serves standard grade process 1. All materials have a sheet thickness of 2.50 mm, the results apply to the test according to ISO 16630. It can be seen that the steel according to the invention achieve better expansion values in the case of punched holes than the standard grades with same processing. Process 1 corresponds hereby to an annealing for example to a hot dip galvanization with combined directly fired furnace and radiant tube furnace, as described in FIG. 9b. The process 2 corresponds for example to a process sequence in a continuous annealing system, as described in FIG. 9c. In addition in this case a reheating of the steel by means of an induction furnace can optionally be achieved immediately prior to the galvanizing bath. As a result of the different temperature courses according to the invention within the mentioned range, different characteristic values result or also a different hole expansion results which are both significantly improved compared to the standard grades. A principal difference are thus the temperature-time parameters in the heat treatment and the downstream cooling.
(19) FIG. 3: shows the relevant alloy elements of the steel according to the invention compared to standard grade, which exemplifies the state of the art. In the comparison steel (standard grade) which corresponds to the state of the art, the main difference is in the carbon content, which lies in the hyper-peritectic range, but also in the elements silicone, manganese and chromium. In addition the standard grade is micro-alloyed with phosphorous. The steels according to the invention are micro alloyed with niobium and have a significantly increased manganese content.
(20) FIG. 4a: shows the mechanical characteristic values transversely and longitudinally to the rolling direction of the steel according to the invention for example in its variant 1, 2 and 3 compared to the standard grade which exemplifies the state of the art. All characteristic values, which were achieved by annealing in the dual phase region, correspond to the normative guidelines of a HCT600X.
(21) FIG. 4b: shows the mechanical characteristic values transversely to the rolling direction of the steel according to the invention exemplary in its variants 1, 2 and 3 which was determined via a regression calculation. Shown are the mechanical characteristic values depending on the manganese content variation depending on the strip thickness (invention variants 1, 2 and 3). All characteristic values correspond to the normative guidelines. The yield ultimate ratio is significantly below 67% for all variants.
(22) FIG. 4d: shows the mechanical characteristic values transversely to the rolling direction and the chemical composition of the steel according to the invention (variant 1) in case of a material thickness or 2.50 mm and an annealing above Ac3. All characteristic values correspond to the normative guidelines of HDT580X.
(23) FIG. 5: schematically shows the time temperature course of the process steps hot rolling and continuous annealing of strips made of the alloy composition according to the invention. Shown is the time and temperature dependent transformation for the hot rolling process as well as for a heat treatment after the cold rolling, exemplary for variant 1.
(24) FIG. 6: shows a schematic ZTU diagram for the steel according to the invention, differentiated according to variant 1, 2 and 3. Herein the determined ZTU diagram is shown with the corresponding chemical composition (variation of exclusively contents of manganese) and the Ac1 and Ac3 temperature. By adjusting corresponding temperature time course during the cooling s wide spectrum of microstructure compositions can be advantageously adjusted. Of particular interest is here also the shifting of the ferrite nose, perlite nose and bainite nose toward later times in the graded increase of manganese contents, this enables the potential to adjust similar microstructure proportions over the entire thickness spectrum in a system speed which depends on the strip thickness.
(25) FIG. 7: shows the mechanical characteristic values longitudinally to the rolling direction with same parameters of continuously annealed strips when varying the rolling reduction degrees or different strip thickness when form example observing variant 1. Shown are the characteristic values tensile strength, yield strength and elongation at break in dependence on selected rolling reduction degrees. Only the tensile strength increases with increasing rolling reduction degrees. All values up to 30% rolling reduction degrees are in the range of the norm for HCT600X. Higher rolling reduction degrees (greater than 75%) lead to the steel grade shift toward HCT780X with minimal strengths of 780 MPa.
(26) FIG. 8: shows an overview over the strength classes that can be adjusted with the alloy concept according to the invention (variant 1). The used alloy composition corresponds to the one shown in FIG. 3. Shown are the differently processed steel strips with their characteristic values longitudinally to the rolling direction and microstructure compositions. This illustrates the range of adjustable strength classes for hot and cold strips with the resulting microstructure proportions depending of the performed process steps and the adjusted process parameters.
(27) FIG. 9 schematically show the temperature time courses in the annealing treatment and cooling with three different variants and in each case different austenizing conditions corresponding to the applied for claims to the method.
(28) The method 1 (FIG. 9a) shows the annealing and cooling of produced cold or hot rolled steel strip in a continuous annealing system. First the strip is heated to a temperature in the range of about 700 950 C. the annealed steel strip ins subsequently cooled from the annealing temperature to an intermediate temperature of about 200 to 250 C. with a cooling rate between about 15 and 100 C./s, a second intermediate temperature (about 300 to 500 C.) is not shown in this schematic representation. Subsequently the steel strip is cooled at air until reaching room temperature with a cooling rate between about 2 and 30 C./s or the cooling with a cooling rate between about 15 and 100 C./s until reaching room temperature is maintained.
(29) The method 2 (FIG. 9b) shows the process according to method 1, however the cooling is briefly interrupted for the purpose of a hot dip galvanizing when passing through the hot dip container and is continued with a cooling rate between about 15 and 100 C./s until reaching an intermediate temperature of about 200 to 250 C. subsequently the steel strip is cooled at air with a cooling rate between about 2 and 30 C./s until reaching room temperature.
(30) The method 3 (FIG. 9c) also shows the process according to method 1 in a hot dip coating, however the cooling of the steel strip is interrupted by a brief brake (about 1 to 20 s) at an intermediate temperature in the range of about 200 to 250 C. and reheated to a temperature which is required for hot dip coating (about 420 to 470 C.). Subsequently the steel strip is cooled again until reaching an intermediate temperature of about 200 to 250 C. The final cooling of the steel strip to room temperature occurs at air with a cooling rate of about 2 and 30 C./s
