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

Abstract

A high-strength air-hardenable multiphase steel having minimal tensile strengths in a non air hardened state of 750 MPa and excellent processing properties, said steel comprising the following elements in % by weight: C0.075 to 0.115; Si0.200 to 0.300; Mn1.700 to 2.300; Cr0.280 to 0.4800; Al0.020 to 0.060; N0.0020 to 0.0120; S0.0050; Nb0.005 to 0.050; Ti0.005 to 0.050; B0.0005 to 0.0060; 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 the steel a sum content of M+Si+Cr in the 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 2.350 and 2.500%, for strip thicknesses of over 1.00 to 2.00 mm the sum of Mn+Si+Cr is 2.500 and 2.950%, and for strip thicknesses of over 2.00 mm the sum of Mn+Si+Cr is 2.950 and 3.250%.

Claims

1. A method comprising: producing a steel strip from a high-strength air-hardenable multiphase steel having a minimum tensile strength before undergoing air hardening of 750 MPa, said steel comprising the following elements in % by weight: C0.075 to 0.115 Si0.200 to 0.300 Mn1.700 to 2.300 Cr0.280 to 0.4800 Al0.020 to 0.060 N0.0020 to 0.0120 S0.0050 Nb0.005 to 0.050 Ti0.005 to 0.050 B0.0005 to 0.0060 Ca0.0005 to 0.0060 Cu0.050 Ni0.050, remainder iron, including usual steel accompanying smelting related impurities, establishing a widest possible process window during continuous annealing of a hot rolled or cold rolled strip, adjusting a sum content of Mn+Si+Cr in said steel as a function of a thickness of the steel strip to be produced according to the following relationship: for a strip thickness of the steel strip of up to 1.00 mm the sum content of Mn+Si+Cr is 2.350 and 2.500%, for a strip thickness of the steel strip of over 1.00 to 2.00 mm the sum of Mn+Si+Cr is 2.500 and 2.950%, and for a strip thickness of the steel strip of over 2.00 mm the sum of Mn+Si+Cr is 2.950 and 3.250%; cold-rolling or hot-rolling the steel strip, during the continuous annealing of the cold-rolled or hot-rolled steel strip, heating the cold-rolled or hot-rolled steel strip to a temperature in the range from about 700 to 950 C., wherein the heating step is performed using a plant configuration comprising a directly fired furnace and a radiant tube furnace, cooling the annealed steel strip from an annealing temperature to a first intermediate temperature of about 300 to 500 C. with a cooling rate of between about 15and 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 100 C./s from the first intermediate temperature to room temperature, 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.

2. The method of claim 1, wherein for a strip thickness of the steel strip of up to 1.00 mm the C-content is 0.100% and a carbon equivalent CEV (IIW) of the steel is 0.56%.

3. The method of claim 1, wherein for a strip thickness of the steel strip 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 0.59%.

4. The method of claim 1, wherein for a strip thickness of the steel strip of more than 2.00 mm, the C content is 0.115% and a carbon equivalent CEV (IIW) of the steel is 0.62%.

5. The method of claim 1, wherein for a strip thickness of the steel strip of up to 1.00 mm the Mn content is 1.700 to 2.000%.

6. The method of claim 1, wherein for a strip thickness of the steel strip above 1.00 to 2.00 mm the Mn content is 1.850 to 2.150%.

7. The method of claim 1, wherein for a strip thickness of the steel strip above 2.00 mm, the Mn content is 2.000 to 2.300%.

8. The method of claim 1, wherein at a sum of the contents of Ti+Nb+B of 0.010 to 0.050% the N content is 0.0020 to 0.0090%.

9. The method of claim 1, wherein at a sum of the contents of Ti+Nb+B of >0.050% the N content is 0.0040 to 0.0120%.

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

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

12. The method of claim 1, wherein the Ti content is 0.020 to 0.050%.

13. The method of claim 1, wherein the Nb content is 0.020 to 0.040%.

14. The method of claim 1, wherein a sum of the contents of Nb+Ti is 0.01 to 0.100%.

15. The method of claim 1, wherein a sum of the contents of Nb+Ti is 0.01 to 0.090%.

16. The method of claim 1, wherein a sum of the contents of Ti+Nb+B is 0.0105 to 0.106%.

17. The method of claim 1, wherein a sum of the contents of Ti+Nb+B is 0.0105 to 0.097%.

18. The method of claim 1, wherein the Ca content is 0.0005 to 0.0030%.

19. The method 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].

20. The method of claim 1, 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.

21. The method of claim 1, 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 30C./s.

22. The method of claim 1, 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.

23. The method of claim 1, further comprising after the heating and cooling steps skin-passing the steel strip.

24. The method of claim 1, further comprising after the heating and cooling steps stretch leveling the steel strip.

25. A method comprising: producing a steel strip from a high-strength air-hardenable multiphase steel having a minimum tensile strength before undergoing air hardening of 750 MPa, said steel comprising the following elements in % by weight: C0.075 to 0.115 Si0.200 to 0.300 Mn1.700 to 2.300 Cr0.280 to 0.4800 Al0.020 to 0.060 N0.0020 to 0.0120 S0,0050 Nb0.005 to 0.050 Ti0.005 to 0.050 B0.0005 to 0.0060 Ca0.0005 to 0.0060 Cu0.050 Ni0.050, remainder iron, including usual steel accompanying smelting related impurities, and establishing a widest possible process window during continuous annealing of a hot rolled or cold rolled strip, adjusting a sum of content of Mn+Si+Cr in said steel as a function of a thickness of the steel strip to be produced according to the following relationship: for a strip thickness of the steel strip up to 1.00 mm the sum content of Mn+Si+Cr is 2.350 and 2.500%, for a strip of thickness of the steel strip over 1.00 to 2.00 mm the sum of Mn+Si+Cr is 2.500 and 2.950%, and for a strip thickness of the steel strip of over 2.00 mm the sum of Mn+Si+Cr is 2.950 and 3.250%, cold-rolling or hot-rolling the steel strip, during the continuous annealing of the cold-rolled or hot-rolled steel strip, heating the cold-rolled or hot-rolled steel strip to a temperature in the range from about 700 to 950 C., wherein the heating is performed with a single radiant tube furnace, cooling the annealed steel strip from an annealing temperature to a first intermediate temperature of about 300 to 500 C. with a cooling rate of between about 15and 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 100 C./s from the first intermediate temperature to room temperature, 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.

26. The method of claim 25, 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 30C./s.

27. The method of claim 25, 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.

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 and 3b: Chemical composition of the investigated steels;

(6) FIG. 4a: Mechanical characteristic values (along the rolling direction) as target values, in the air-hardened and non-tempered state;

(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 a table depicting the chemical composition of the investigated steels. LH1000 alloys according to the invention and comparing them 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 neither V nor Mo are added.

(18) FIG. 3 is a table showing the sum contents of various alloying components in percent by weight and stating the respectively determined carbon equivalent CEV (IIW).

(19) FIGS. 4a, 4b, 4c show the mechanical characteristic values along the rolling direction of the investigated steels, with the target values to be achieved for the air-hardened state (FIG. 4a), the values determined in the non-air-hardened initial state (FIG. 4b) and in the air-hardened state (FIG. 4c). The values to be achieved are reached by a safe margin.

(20) 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.

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

(22) 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.

(23) 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.

(24) 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.

(25) FIGS. 6a, 6b, 6c schematically show three variants of the temperature-time curves according to the invention during the annealing treatment and cooling and in each case various austenitization conditions.

(26) 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.

(27) 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.

(28) 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.

(29) 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.

(30) 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) Variant A/2.00 mm/Method 2 According to FIG. 6b.

(31) A steel according to the invention with 0.104% C; 0.288% Si; 2.020% Mn; 0.011% P; 0.001% S; 0.0047% N; 0.042 Al; 0.319% Cr; 0.0490% Ti; 0.0388% Nb; 0.0018% B; 0.0012% 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 4.09 mm and after pickling without additional heat treatment (such as batch annealing) cold rolled.

(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 LH1000:

(39) TABLE-US-00001 Yield strength (Rp 0.2) 814 MPa (530 MPa) Tensile strength (Rm) 1179 MPa (855 MPa) Elongation at break (A80) 5.8% (16.1%) A5 elongation 12.9% () Bake Hardening Index (BH2) 58 MPa Hole expansion ratio according to ISO 16630 (21%) Bending angle accord. to VDA 238-100 (longitudinal, transverse) (88/77)

(40) The yield ultimate ratio Re/Rm in the longitudinal direction was 62% in the initial state.

EXAMPLE 2

(Cold Strip) (Alloy Composition in % by Weight) Variant B/2.0 mm/Method 3 According to FIG. 6c.

(41) A steel according to the invention with 0.101% C; 0.273% Si; 1.846% Mn; 0.012% P; 0.001% S; 0.0040% N; 0.036 Al; 0.453% Cr; 0.0295% Ti; 0.0265% Nb; 0.0019% B; 0.0015% 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 coiling target temperature of 650 C. with a thickness of 4.09 mm and after the pickling was cold rolled without additional heat treatment (such as for example batch annealing).

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

(43) Annealing temperature 870 C.

(44) Holding time 120 s

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

(46) Subsequent cooling at air

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

(48) 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 LH1000:

(49) TABLE-US-00002 Yield strength (Rp 0.2) 803 MPa (502 MPa) Tensile strength (Rm) 1113 MPa (815 MPa) Elongation at break (A80) 13.1% (18.9%) A5 elongation 7.1% () Bake Hardening Index (BH2) 53 MPa Hole expansion ratio according to ISO 16630 (31%) Bending angle accord. to VDA 238-100 (longitudinal, transverse) (95/90)

(50) The yield ultimate ratio Re/Rm in the longitudinal direction was 62% in the initial state.