Ultra high-strength air-hardening multiphase steel having excellent processing properties, and method for manufacturing a strip of said steel

Abstract

An ultra-high-strength air-hardenable multiphase steel having minimal tensile strengths in a non air hardened state of 950 MPa and excellent processing properties, includes the following elements in % by weight: C0.075 to 0.115; Si0.400 to 0.500; Mn1,900 to 2,350; Cr0.200 to 0.500; Al0.005 to 0.060; N0.0020 to 0.0120; S0.0030; Nb0.005 to 0.060; Ti0.005 to 0.060; B0.0005 to 0.0030; Mo0.200 to 0.300; Ca0.0005 to 0.0060; Cu0.050; Ni0.050; remainder iron, including usual steel accompanying smelting related impurities, wherein for a widest possible process window during continuous annealing of hot rolled or cold rolled strips made from said steel a sum content of M+Si+Cr in said steel is a function of a thickness of the steel strips according to the following relationship: for strip thicknesses of up to 1.00 mm the sum content of M+Si+Cr is 2.800 and 3.000%, for strip thicknesses of over 1.00 to 2.00 mm the sum of Mn+Si+Cr is 2.850 and 3.100%, and for strip thicknesses of over 2.00 mm the sum of Mn+Si+Cr is 2.900 and 3.200%.

Claims

1. An ultra-high-strength air-hardenable multiphase steel having minimal tensile strengths in a non air hardened state of 950 MPa and excellent processing properties, said steel comprising the following elements in % by weight: C0.075 to 0.115 Si0.400 to 0.500 Mn1,900 to 2,350 Cr0.200 to 0.500 Al0.005 to 0.060 N0.0020 to 0.0120 S0.0030 Nb0.005 to 0.060 Ti0.005 to 0.060 B0.0005 to 0.0030 Mo0.200 to 0.300 Ca0.0005 to 0.0060 Cu0.050 Ni0.050 remainder iron, including usual steel accompanying smelting related impurities, wherein for a widest possible process window during continuous annealing of hot rolled or cold rolled strips made from said steel a sum content of M+Si+Cr in said steel is a function of a thickness of the steel strips according to the following relationship: for strip thicknesses of up to 1.00 mm the sum content of M+Si+Cr is 2.800 and s 3.000%, for strip thicknesses of over 1.00 to 2.00 mm the sum of Mn+Si+Cr is 2.850 and 3.100%, and for strip thicknesses of over 2.00 mm the sum of Mn+Si+Cr is 2.900 and 3.200%.

2. The steel of claim 1, wherein for strip thicknesses of up to 1.00 mm the C-content is 0.100% and a carbon equivalent CEV (IIW) of the steel is 0.62%.

3. The steel of claim 1, wherein for strip thicknesses of more than 1.00 to 2.00 mm, the C-content is 0.105% and a carbon equivalent CEV (IIW) of the steel is 0.64%.

4. The steel of claim 1, wherein for strip thicknesses of more than 2.00 mm, the C content is 0.115% and a carbon equivalent CEV (IIW) of the steel is 0.66%.

5. The steel of claim 1, wherein for strip thicknesses of up to 1.00 mm the Mn content is 1.900 to 2.200%.

6. The steel of claim 1, wherein for strip thicknesses above 1.00 to 2.00 mm the Mn content is 2.050 to 2.250%.

7. The steel of claim 1, wherein for strip thicknesses above 2.00 mm, the Mn content is 2.100 to 2.350%.

8. The steel of claim 1, wherein at a sum of the contents of Ti+Nb+B of 0.010 to 0.070% the N content is 0.0020 to 0.0090%.

9. The steel of claim 1, wherein at a sum of the contents of Ti+Nb+B of >0.070% the N content is 0.0040 to 0.0120%.

10. The steel of claim 1, wherein the S content is 0.0025%.

11. The steel of claim 1, wherein the S content is 0.0020%.

12. The steel of claim 1, wherein the Mo content is 0.250%.

13. The steel of claim 1, wherein the Ti content is 0.025 to 0.045%.

14. The steel of claim 1, wherein the Nb content is 0.025 to 0.045%.

15. The steel of claim 1, wherein a sum of the contents of Nb+Ti is 0.100%.

16. The steel of claim 1, wherein a sum of the contents of Nb+Ti is 0.090%.

17. The steel of claim 1, wherein a sum of the contents of Cr+Mo is 0.725%.

18. The steel of claim 1, wherein a sum of the contents of Ti+Nb+B is 0.102%.

19. The steel of claim 1, wherein a sum of the contents of Ti+Nb+B is 0.092%.

20. The steel of claim 1, wherein a sum of the contents of Ti+Nb+B+Mo+V is 0.365%.

21. The steel of claim 1, wherein the Ca content is 0.0030%.

22. The steel of claim 1, wherein the contents of silicon and manganese with respect to strength properties to be achieved are interchangeable according to the relationship:
YS (MPa)=160.7+147.9[% SI]+161.1[% Mn]
TS (MPa)=324.8+189.4[% Si]+174.1[% Mn].

23. A method for producing a cold-rolled or hot-rolled steel strip from a multi-phase, air-hardenable steel comprising the following elements in % by weight: C0.075 to 0.115 Si0.400 to 0.500 Mn1,900 to 2,350 Cr0.200 to 0.500 Al0.005 to 0.060 N0.0020 to 0.0120 S0.0030 Nb0.005 to 0.060 Ti0.005 to 0.060 B0.0005 to 0.0030 Mo0.200 to 0.300 Ca0.0005 to 0.0060 Cu0.050 Ni0.050, remainder iron, including usual steel accompanying smelting related impurities, wherein for a widest possible process window during continuous annealing of hot rolled or cold rolled strips made from said steel a sum content of M+Si+Cr in said steel is a function of a thickness of the steel strips according to the following relationship: for strip thicknesses of up to 1.00 mm the sum content of M+Si+Cr is 2.800 and 3.000%, for strip thicknesses of over 1.00 to 2.00 mm the sum of Mn+Si+Cr is 2.850 and 3.100%, and for strip thicknesses of over 2.00 mm the sum of Mn+Si+Cr is 2.900 and 3.200%, said method comprising: continuously annealing the multi-phase, air-hardenable steel to produce a cold-rolled or hot-rolled steel strip; heating the cold-rolled or hot-rolled steel strip during the continuous annealing to a temperature in the range from about 700 to 950 C.; cooling the annealed steel strip from the annealing temperature to a first intermediate temperature of about 300 to 500 C. with a cooling rate of between about 15 and 100 C./s; and after the cooling to the intermediate temperature treating the steel strip as set forth under a) or b): a) cooling the steel strip to a second intermediate temperature of about 160 to 250 C. with a cooling rate of between 15 and 100 C./s and after cooling to the second intermediate temperature cooling the steel strip at air to room temperature; b) maintaining the cooling of the steel strip with a cooling rate of between about 15 and 100C/s from the first intermediate temperature to room temperature.

24. The method of claim 23, further comprising after the heating step and during the cooling to the first intermediate temperature step hot dip coating the steel strip in a hot dip bath, wherein the cooling to the first intermediate temperature is interrupted prior to entry into the hot dip bath, and after the cooling to the first intermediate temperature the steel strip is treated as set forth under a), wherein the second intermediate temperature is 200 to 250 C. and the cooling from the second intermediate temperature to room temperature is conducted with a cooling rate of about 2 and 30 C./s.

25. The method of claim 23, wherein the steel strip is treated as set forth under a), wherein the second intermediate temperature is 200 to 250 C., said method further comprising after the cooling to the second intermediate temperature and prior to the cooling to room temperature, holding the second intermediate temperature for about 1 to 20 seconds, reheating the steel strip to a temperature of about 400 to 470 C., hot dip coating the steel strip, and cooling the steel strip to the second intermediate temperature of 200 to 250 C. with a cooling rate of between about 15 and 100 C./s, wherein the cooling from the second intermediate temperature to room temperature is conducted with a cooling rate of about 2 and 30 C./s.

26. The method of claim 23, wherein the heating step is performed using a plant configuration comprising a directly fired furnace and a radiant tube furnace, and wherein the method further comprises increasing an oxidation potential during the heating by setting a CO-content in the directly fired furnace below 4%, setting an oxygen partial pressure of an atmosphere of the radiant tube furnace according to the following equation,
18>Log pO.sub.25*Si.sup.0.32.2*Mn.sup.0.450.1*Cr.sup.0.412.5*(InB).sup.0.25, wherein Si, Mn, Cr and B are corresponding alloy proportions in the steel in % by weight and pO.sub.2 is the oxygen partial pressure in mbar, and wherein a dew point of an overall atmosphere of the plant configuration is set to 30 C. or below for avoiding oxidation of the strip directly prior to immersion into a hot dip bath.

27. The method of claim 23, wherein the heating is performed with a single radiant tube furnace, and wherein an oxygen partial pressure of an atmosphere of the radiant tube furnace satisfies the following equation,
12>Log pO.sub.25*Si.sup.0.253*Mn.sup.0.50.1*Cr.sup.0.57*(InB).sup.0.5 wherein Si, Mn, Cr, and B are corresponding alloy components in the steel in % by weight and pO.sub.2 is the oxygen partial pressure in mbar, and wherein a dew point of an overall atmosphere of the plant configuration is set to 30 C. or below for avoiding oxidation of the strip directly prior to immersion into a hot dip bath.

28. The method of claim 23, further comprising adjusting a plant throughput speed to different thicknesses of respective steel strips so that heat treatment of the respective steel strips results in similar microstructures and mechanical characteristic values.

29. The method of claim 23, further comprising after the heating and cooling steps skin-passing the steel strip.

30. The method of claim 23, further comprising after the heating and cooling steps stretch leveling the steel strip.

31. A steel strip produced by the method of claim 23 and having a minimum hole-expansion value according to ISO 16630 of 20% in a non-air-hardened state.

32. The steel strip of claim 31, having a minimum hole-expansion value according to ISO 16630 of 25% in a non-air-hardened state.

33. The steel strip of claim 31, having a minimum bending angle according to VDA 238-100 of 50 in a longitudinal direction or transverse direction in a non-air-hardened state.

34. The steel strip of claim 31, having a minimum bending angle according to VDA 238-100 of 65 in a longitudinal direction or transverse direction in a non-air-hardened state.

35. The steel strip of claim 31, having a minimum product value tensile strength Rm x bending angle according to VDA 238-100 of 100,000 MPa in a non-air-hardened state.

36. The steel strip of claim 31, having a minimum product value tensile strength Rm x bending angle according to VDA 238-100 of 120,000 MPa in the non-air-hardened state.

37. The steel strip of claim 31, having a delayed fracture free state for at least 6 months, meeting the requirements of SEP 1970 for perforated tensile and bent beam test.

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 a drawing.

(2) It is shown in:

(3) FIG. 1: Process chain (schematic) for the production of a strip from the steel according to the invention

(4) FIG. 2: Time-temperature profile (schematic) of the process steps of hot-rolling and cold-rolling (optional) and continuous annealing, component manufacturing, heat treatment (air hardening) and tempering (optional) exemplary for the steel according to the invention

(5) FIGS. 3a, 3b: Chemical composition of the investigated steels

(6) FIG. 4a: Mechanical characteristic values (along the rolling direction) as target values, air-hardened and not tempered

(7) FIG. 4b: Mechanical characteristic values (along the direction of rolling) of the stepped steels in the initial state

(8) FIG. 4c: Mechanical characteristic values (along the rolling direction) of the steered steels in the air-hardened, non-tempered state

(9) FIG. 5: Results of the hole spreading tests according to ISO 16630 and the plate bending test according to VDA 238-100 on steels according to the invention

(10) FIG. 6a: Method 1, temperature-time curves (annealing variants schematically)

(11) FIG. 6b: Method 2, temperature-time curves (annealing variants schematically)

(12) FIG. 6c: Method 3, temperature-time curves (annealing variants schematically)

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(13) FIG. 1 shows a schematic illustration of the process chain for producing a strip from the steel according to the invention. The various process routes pertaining to the invention are illustrated. Until the hot rolling (final rolling temperature), the process route is the same for all steels according to the invention, afterwards, depending on the desired results, different process routes take place. For example, the pickled hot strip can be galvanized or cold-rolled and galvanized with different degrees of rolling. It is also possible to cold-rolled and galvanized hot-annealed hot-rolled strip or soft-annealed cold strip.

(14) Optionally it is also possible to process material without hot dip refining, i.e., only by continuous annealing with and without subsequent electrolytic galvanizing. A complex component can now be produced from the optionally coated material. Subsequently, the hardening process takes place, in which cooling is performed at air in accordance with the invention. Optionally, a tempering stage can complete the temperature treatment of the component.

(15) FIG. 2 schematically shows the time-temperature profile of the process steps hot rolling and continuous annealing of strips made from the alloy composition according to the invention. The time and temperature-dependent transformation for the hot-rolling process as well as for a heat treatment after cold-rolling, component production, quenching and tempering and optional tempering are shown.

(16) FIG. 3a shows the chemical composition of the investigated steels. LH1100 alloys according to the invention were compared with the reference grades LH800/LH900.

(17) Compared to the reference grades, the alloys according to the invention have, in particular, significantly increased contents of Si and lower contents of Cr and no added V.

(18) FIG. 3b shows the sum contents of various alloying components in percent by weight and the respectively determined carbon equivalent CEV (IIW) is stated.

(19) FIG. 5 shows results of the hole expansion tests according to ISO 16630 (absolute values). The results of the hole expansion tests for variant A (coiling temperature above bainite starting temperature) for process 2 (FIG. 6b, 1.2 mm) and process 3 (FIG. 6c, 2.0 mm) are shown.

(20) The investigated materials have a sheet thickness of 1.2 and 2.0 mm, respectively. The results apply to the test according to ISO 16630.

(21) Method 2 corresponds for example to an annealing on a hot galvanizing with a combined direct-fired furnace and a radiant tube furnace as described in FIG. 6b.

(22) Method 3 corresponds, for example, to a process control in a continuous annealing system as described in FIG. 6c. In addition, by means of an induction furnace, a reheating of the steel can be achieved in this case directly before the zinc bath.

(23) The different temperature profiles according to the invention within the mentioned range, result in different characteristic values or also different hole expansion results as well as bending angles. The principal differences are thus the temperature-time parameters during the heat treatment and the subsequent cooling.

(24) FIG. 6 schematically shows three variants of the temperature-time curves according to the invention during the annealing treatment and cooling and in each case various austenitization conditions.

(25) Method 1 (FIG. 6a) shows the annealing and cooling of the produced cold-rolled or hot-rolled or cold-re-rolled steel strip in a continuous annealing line. First, the strip is heated to a temperature in the range of about 700 to 950 C. (Ac1 to Ac3). The annealed steel strip is then cooled from the annealing temperature to an intermediate temperature (IT) of about 200 to 250 C. at a cooling rate between about 15 and 100 C./sec. A second intermediate temperature (about 300 to 500 C.) is not shown in this schematic illustration.

(26) Subsequently, the steel strip is cooled at air at a cooling rate of between about 2 and 30 C./sec until room temperature (RT) is reached, or the cooling to room temperature is maintained at a cooling rate of between about 15 and 100 C./sec.

(27) Method 2 (FIG. 6b) shows the process according to method 1, however, for the purpose of hot dip finishing the cooling of the steel strip is intermittently interrupted during the passage through the hot dip vessel to then cool to an intermediate temperature of about 200 to 250 C. at a cooling rate of between about 15 and 100 C./s. Subsequently, the steel strip is cooled at air at a cooling rate of between about 2 and 30 C./sec until room temperature is reached.

(28) Method 3 (FIG. 6c) also shows the process according to method 1 in the case of a hot dip refining, but the cooling of the steel strip is interrupted by a short pause (about 1 to 20 s) at an intermediate temperature in the range of approx. 200 to 400 C. and reheated to the temperature (ST) necessary for the hot dip immersion (about 400 to 470 C.). Subsequently, the steel strip is cooled again to an intermediate temperature of approximately 200 to 250 C. With a cooling rate of approx. between 2 and 30 C./s, the final cooling of the steel strip takes place at air until room temperature is reached.

(29) The following examples are used for industrial production for the hot-dip galvanizing according to method 2 according to FIG. 6b and according to method 3 according to FIG. 6c with a lab-borated coating process:

Example 1 (Cold Strip) (Alloy Composition in % by Weight)

(30) Variant A/1.2 mm/Method 2 According to FIG. 6b

(31) A steel according to the invention with 0.099% C; 0.461% Si; 2.179% Mn; 0.009% P; 0.001% S; 0.0048% N; 0.040 Al; 0.312% Cr; 0.208% Mo; 0.0292% Ti; 0.0364% Nb; 0.0012% B; 0.0015% Ca hot dip refined according to method 2 according to FIG. 6b, the material was hot-rolled beforehand at a final rolling target temperature of 910 C. and coiled at a final rolling target temperature of 650 C. with a thickness of 2.30 mm and after pickling without additional heat treatment (such as annealing) cold rolled twice with an intermediate thickness of 1.49 mm.

(32) In an annealing simulator, a hot dip refined, air-hardened steel strip was processed with the following parameters

(33) Annealing temperature 870 C.

(34) Holding time 120 s

(35) Transport time max. 5 s (without energy input)

(36) Subsequent cooling at air

(37) After tempering, the steel according to the invention has a microstructure consisting of martensite, bainite and residual austenite.

(38) This steel shows the following characteristic values after air hardening (initial values in brackets, unprocessed condition) Along the rolling direction, and would correspond, for example, to an LH1100:

(39) TABLE-US-00001 Yield strength (Rp0.2) 921 MPa (768 MPa) Tensile strength (Rm) 1198 MPa (984 MPa) Elongation at break (A80) 5.5% (10.7%) A5 elongation 9.5% () Bake Hardening Index (BH2) 52 MPa Hole expansion ratio according to ISO 16630 (49%) Bending angle accord. to VDA 238-100 (122/112) (longitudinal, transverse)

(40) The yield ultimate ratio Re/Rm in the longitudinal direction was 78% in the initial state.

Example 2 (Cold Strip) (Alloy Composition in % by Weight)

(41) Variant B/2.0 mm/Method 3 According to FIG. 6c

(42) A steel according to the invention with 0.100% C; 0.456% Si; 2.139% Mn; 0.010% P; 0.001% S; 0.0050% N; 0.058 Al; 0.313% Cr; 0.202% Mo; 0.0289% Ti; 0.0337% Nb; 0.0009% B; 0.0021% Ca hot dip refined according to method 3 according to FIG. 6c, the material was subjected beforehand to hot rolling at a final rolling target temperature of 910 C. and was coiled at a core coiling temperature of 650 C. with a thickness of 2.30 mm and after the pickling was cold rolled without additional heat treatment (such as, batch annealing).

(43) In an annealing simulator, the hot dip refined steel was processed with the following parameters analogous to a temperature treatment process (air-hardening):

(44) Annealing temperature 870 C.

(45) Holding time 120 s

(46) Transport time max. 5 s (without energy input)

(47) Subsequent cooling at air

(48) After tempering, the steel according to the invention has a microstructure consisting of martensite, bainite and residual austenite.

(49) This steel shows the following characteristic values after air hardening (initial values in brackets, unprocessed condition) along the rolling direction, and would correspond, for example, to an LH1100:

(50) TABLE-US-00002 Yield strength (Rp0.2) 903 MPa (708 MPa) Tensile strength (Rm) 1186 MPa (983 MPa) Elongation at break (A80) 7.1% (11.7%) A5 elongation 9.1% () Bake Hardening Index (BH2) 48 MPa Hole expansion ratio according to ISO 16630 (32%) Bending angle accord. to VDA 238-100 (longitudinal, transverse) (104/88)

(51) The yield ultimate ratio Re/Rm in the longitudinal direction was 72% in the initial state.