STEEL STRIP MADE OF A HIGH-STRENGTH MULTIPHASE STEEL AND PROCESS FOR PRODUCING SUCH A STEEL STRIP

Abstract

A steel strip made of a high-strength multiphase steel with a tensile strength of at least 780 MPa in the longitudinal direction and consists of the following elements in % by weight: 0.08?C?0.23, 1.5?Mn?3.5, 0.25?Si+Al?2, 0.0020?N?0.0160, P<0.05, S<0.01. Cu<0.20; iron; and a carbon equivalent CEV which is greater than 0.49 and smaller than 0.9, wherein the carbon equivalent CEV results from the contents of the corresponding elements in % by weight according to the following formula: CEV=C+Mn/6 (Cu+Ni)/15+(Cr+Mo+V)/5 and wherein the ratio of the carbon equivalent CEV and the sum of the contents of Si and Al in % by weight is less than 2.3, wherein the multiphase steel constituents martensite, tempered martensite, residual austenite, upper bainite and/or lower bainite where the sum of the volume fractions of the microstructure is at least 30% by volume, and residual microstructure consists of ferrite and perlite.

Claims

1. A steel strip consisting of a high-strength multiphase steel which has a tensile strength of at least 780 MPa in the longitudinal direction, the multiphase steel consisting of the following elements in wt. %: TABLE-US-00007 C ? 0.08 to ? 0.23, Mn ? 1.5 to ? 3.5, Si + Al ? 0.25 to ? 2, N ? 0.0020 to ? 0.0160, P < 0.05, S < 0.01, Cu < 0.20, optionally one or more of the following elements: TABLE-US-00008 Ca ? 0.0005 to ? 0.0060, Cr ? 0.05 to ? 1.0, Mo ? 0.05 to ? 1.0, Ni ? 0.05 to ? 0.50, Nb ? 0.005 to ? 0.15, Ti ? 0.005 to ? 0.15, V ? 0.001 to ? 0.30 and B ? 0.0005 to ? 0.0050 with the remainder being iron including typical steel-associated, melting-induced impurities, and having a carbon equivalent CEV which is greater than 0.49 and less than 0.9, preferably greater than 0.49 and less than 0.75, wherein the carbon equivalent CEV is determined according to the formula
CEV=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 from the contents of the corresponding elements in wt. % and wherein the ratio of the carbon equivalent CEV and the sum of the contents of Si and Al in wt. % is less than 2.3, wherein the multiphase steel has a microstructure, in which the sum of the volume proportions of the microstructure constituents of martensite, tempered martensite, residual austenite, upper bainite and/or lower bainite is at least 30 vol. % and the remaining microstructure consists of ferrite and pearlite.

2. The steel strip as claimed in claim 1, wherein the ratio of elasticity limit to tensile strength R.sub.p0.2/R.sub.m is less than 0.8 and the elongation at fracture A.sub.80 is >5%.

3. The steel strip as claimed in claim 1, wherein the content in wt. % of the element C is between 0.09 and 0.2 and/or the content in wt. % of the element Mo is less than 0.4.

4. The steel strip as claimed in claim 1, wherein the content in wt. % of the element Mn is between 1.8 and 2.5 and/or that the content in wt. % of the sum of the elements Si+Al is between 0.25 and 1.

5. The steel strip as claimed in claim 1, wherein the carbon equivalent CEV is less than 0.7.

6. The steel strip as claimed in claim 1, wherein the sum of the volume proportions of the microstructure constituents of martensite, tempered martensite, residual austenite, upper bainite and/or lower bainite in the microstructure of the multiphase steel is at least 50 vol. %, preferably at least 70 vol. %, and the remaining microstructure consists of ferrite and pearlite.

7. The steel strip as claimed in claim 1, wherein the steel strip has a thickness which specifically varies in the longitudinal extension, wherein the ratio between maximum thickness and minimum thickness is, in particular, between 1.16 and 3.

8. The steel strip as claimed in claim 1, wherein the thickness D of the steel strip is in the range ?4 mm to ?18 mm.

9. A method for producing a steel strip consisting of a high-strength multiphase steel, in particular a steel strip, as claimed in claim 1 which has a tensile strength of at least 780 MPa in the longitudinal direction, wherein a rolled strip sheet of steel consisting of the following elements in wt. %: TABLE-US-00009 C ? 0.08 to ? 0.23, Mn ? 1.5 to ? 3.5, Si + Al ? 0.25 to ? 2. N ? 0.0020 to ? 0.0160, P < 0.05, S < 0.01, Cu < 0.20, optionally one or more of the following elements: TABLE-US-00010 Ca ? 0.0005 to ? 0.0060, Cr ? 0.05 to ? 1.0, Mo ? 0.05 to ? 1.0, Ni ? 0.05 to ? 0.50, Nb ? 0.005 to ? 0.15, Ti ? 0.005 to ? 0.15, V ? 0.001 to ? 0.30 and B ? 0.0005 to ? 0.0050 with the remainder being iron including typical steel-associated, melting-induced impurities, and having a carbon equivalent CEV which is greater than 0.49 and less than 0.9, preferably greater than 0.49 and less than 0.75, wherein the carbon equivalent CEV is determined according to the formula
CEV=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 from the contents of the corresponding elements in wt. % and wherein the ratio of the carbon equivalent CEV and the sum of the contents of Si and Al in wt. % is less than 2.3, is heat-treated as a wholein particular rolled up into a coilsuch that it assumes a temperature above 750? C. and after this heat treatment is cooled to a temperature below 200? ? C., wherein the cooling between 750? C. and 200? ? C. is effected at an average cooling rate greater than 1 K/h and less than 300 K/h.

10. The method as claimed in claim 9, wherein the strip sheet is heated during the heat treatment from 100? C. to a temperature of 750? C. at an average heating rate between 1 K/h and 300 K/h.

11. The method as claimed in claim 9 wherein the strip sheet remains for at least 1 h in the temperature range of 750? C. to Ar.sub.3+70? C., wherein the numerical value of the temperature Ar.sub.3 is calculated by means of the following formula from the contents of the corresponding elements in wt. %:
Ar.sub.3=910?203?{square root over (C)}?30Mn+44.7Si?11Cr+31.5Mo?15.2Ni

12. The method as claimed in claim 9, wherein the strip sheet consisting of steel reaches, during the heat treatment, a maximum temperature of at least 780? C. and at most 900? ? C., preferably of at least 790? C. and at most 850? ? C.

13. The method as claimed in claim 9, wherein the steel strip is provided with a surface coating in the form of a metallic coating, organic coating or lacquer after cooling.

14. The method as claimed in claim 11, wherein the steel strip has a thickness which specifically varies in the longitudinal extension, wherein the ratio between maximum thickness and minimum thickness is, in particular, between 1.16 and 3.

15. The method as claimed claim 12, wherein the strip sheet remains for at least 1 h in the temperature range of 750? ? C. to Ar.sub.3+70? C., wherein the numerical value of the temperature Ar.sub.3 is calculated by means of the following formula from the contents of the corresponding elements in wt. %:
Ar.sub.3=910?203?{square root over (C)}?30Mn+44.7Si?11Cr+31.5Mo?15.2Ni

16. The method as claimed in claim 10, wherein the strip sheet consisting of steel reaches, during the heat treatment, a maximum temperature of at least 780? ? C. and at most 900? C., preferably of at least 790? ? C. and at most 850? C.

17. The method as claimed in claim 16, wherein the steel strip is provided with a surface coating in the form of a metallic coating, organic coating or lacquer after cooling.

18. A method of using a steel strip as claimed in claim 1 for producing a motor vehicle component.

19. The Steel strip as claimed in claim 2, wherein the content in wt. % of the element C is between 0.09 and 0.2 and/or the content in wt. % of the element Mo is less than 0.4.

20. The steel strip as claimed in claim 9, wherein the content in wt. % of the element Mn is between 1.8 and 2.5 and/or that the content in wt. % of the sum of the elements Si+Al is between 0.25 and 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] FIG. 1 is a graphical illustration of the temperature progression over time of a rolled strip sheet consisting of steel and an installation which heat-treats this strip sheet during a heat treatment according to a preferred embodiment of the invention in a temperature-time graph; and

[0068] FIG. 2 are graphical representations of stress-strain curves of a steel strip configured in accordance with the invention and the stress-strain curves of a comparative steel strip having a different composition of the steel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0069] Basically, the annealing treatments in accordance with the invention can be multi-stage or additional annealing treatments can also be provided in relation to the entire process. An exemplary time-temperature cycle which represents the characteristic temperature ranges for retention times, cooling rates and heating rates is specified in FIG. 1.

[0070] For this purpose, a rolled strip sheet consisting of steel of a corresponding composition is put into a compact form, in particular rolled into a coil, which makes it possible to transport the strip sheet as a whole to an apparatus for heat treatment. At this location, in a first step S1 the sheet strip is heated to a temperature T? 750? C. within about 3 h. Subsequently, in a second step S2 the strip sheet is held at a temperature above 750? C. for about 8 h by means of the apparatus. The following applies to the maximum temperature reached in the second step S2: T.sub.max<Ar.sub.3+70 K. The strip sheet is then cooled. During this cooling, the temperature passes through the temperature range of 750? C. to 200? C. in a time period of about 14 h. This gives rise to a third step S3 of cooling from 750? C. to 200? C. at an average cooling rate of about 40 K/h. When the strip sheet consisting of steel of a corresponding steel concept, i.e. a suitable composition, is cooled, the desired microstructure is produced and the steel strip consisting of high-strength multiphase steel is produced. The cooling is effected to a specific temperature preferably in the apparatus for heat treatment. This is e.g. a batch-type annealing installation. The example shown, at about 40 K/h, is in a preferred cooling range of 20 K/h to 80 K/h.

[0071] Material concepts, more specifically steel concepts, and their chemical composition in wt. % are listed by way of example in the following table 1. Steel concepts in accordance with the invention are characterised accordingly. In addition to the steel concepts in accordance with the invention which, in the form of a hot-rolled or cold-rolled strip sheet, are used as input material for production in accordance with the invention of a product in accordance with the invention, steel concepts are likewise indicated as a comparison but are not in accordance with the invention.

[0072] The parameters of a production method in accordance with the invention and the characteristic values of the inventive product of this production method, i.e. the steel strip consisting of high-strength multiphase steel, are listed in Table 2. The exemplary material concepts are explained hereinafter. They are designated as Steel A, Steel B, Steel C and Steel D.

[0073] Steel A is not in accordance with the invention because the sum of alloy elements which increase thorough hardenability, as described by the CEV, is below the required value of 0.49. After a heat treatment involving the process parameters in accordance with the invention, steel A has a microstructure consisting of ferrite and pearlite and no proportions of bainite and/or martensite are formed. The associated stress-strain curve can be seen in FIG. 2. The heat-treated steel A has a tensile strength of 540 MPa and an undesired pronounced yield strength.

[0074] Steel B from Table 2 is likewise not in accordance with the invention, although the steel B, with a CEV value of 2.34, has sufficient thorough hardenability. However, the ratio of CEV/(Si+Al) is >2.34 and thus the content of Si and Al in relation to the use of elements which increase thorough hardenability is not sufficient. This is also apparent from the achievable maximum tensile strengths of 762 MPa.

[0075] By way of example, steels C and D are material concepts which are suitable for an annealing treatment in accordance with the invention and for the production of steel strips produced in accordance with the invention. After a heat treatment involving the process parameters in accordance with the invention, the steels C and D have a martensite and/or bainite proportion of over 30%. By reason of the microstructure set in this way, the steels also have the material properties characteristic of multiphase steels, such as an elasticity limit-tensile strength ratio (R.sub.p0.2/R.sub.m) between 0.45 and 0.6, a high tensile strength R.sub.m above 780 MPa and, at the same time, a high elongation at fracture A.sub.80>8%.

TABLE-US-00005 TABLE 1 Steel C Si Mn P S Cu N Al Cr Mo A 0.146 0.381 1.762 0.013 0.002 0.078 0.0048 0.044 0.043 0.004 B 0.092 0.264 1.908 0.016 0.002 0.016 0.0058 0.033 0.778 0.201 C 0.106 0.492 2.235 0.010 0.001 0.016 0.0063 0.042 0.313 0.212 D 0.098 0.479 2.167 0.011 0.001 0.014 0.0054 0.044 0.319 0.23 CEV/ A.sub.R3 (Si + Inven- Steel Ti V Nb B Ca CEV [? C.] Al) tion A 0.0022 0.093 0.043 0.0002 0.0015 0.4756 796 1.23 No B 0.0297 0.062 0.0036 0.0031 0.0014 0.6216 800 2.34 No C 0.0332 0.004 0.0347 0.0005 0.0014 0.5902 802 1.87 Yes D 0.0278 0.004 0.0374 0.0004 0.0015 0.5728 806 1.89 Yes Ar.sub.3 = 910 ? 203?C ? 30 Mn + 44.7 Si ? 11 Cr + 31.5 Mo ? 15.2Ni CEV = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5

TABLE-US-00006 TABLE 2 Phase proportion Phase consisting of Average Average proportion martensite, heating cooling consisting bainite and rate from Retention rate from of ferrite residual 100 to Maximum time above 750? C. to R.sub.P0, 2 R.sub.m A.sub.80 Rp.sub.0, 2/R.sub.m and pearlite austenite* Steel 750? C. temperature 750? C. 200? C. [MPa] [MPa] [%] [] [%] [%] A 160 K/h 820? C. 5 hours 16.7 K/h 420 555 24.5 0.76 100 A 166 K/h 850? C. 5 hours 16.7 K/h 371 540 23.1 0.69 100 B 160 K/h 820? C. 5 hours 16.7 K/h 332 707 20.4 0.47 70 30 B 166 K/h 850? C. 5 hours 16.7 K/h 447 762 14.3 0.59 60 40 C 160 K/h 820? C. 5 hours 16.7 K/h 461 863 15.7 0.53 60 40 C 166 K/h 850? C. 5 hours 16.7 K/h 427 828 15.5 0.52 65 35 D 160 K/h 820? C. 5 hours 16.7 K/h 503 892 14.7 0.56 55 45 D 166 K/h 850? C. 5 hours 16.7 K/h 465 832 8.9 0.56 60 40 *includes both tempered and also non-tempered phase constituents