Micro-alloyed high-strength multi-phase steel containing silicon and having a minimum tensile strength of 750 MPA and improved properties and method for producing a strip from said steel

Abstract

A high-strength multi-phase steel having minimum tensile strengths of 750 MPa and preferably having a dual-phase microstructure for a cold- or hot-rolled steel strip, in particular for lightweight vehicle construction is disclosed. The high-strength multi-phase steel has improved forming properties and a ratio of yield point to tensile strength of at most 73%. The high-strength multi-phase steel includes in mass %: C0.075 to 0.105; Si0.600 to 0.800; Mn1.000 to 0.700; Cr0.100 to 0.480; Al0.010 to 0.060; N 0.00200.0120; S0.0030; Nb0.005 to 0.050; Ti0.0050 to 0.050; B0.0005 to 0.0040; Mo0.200; Cu0.040%; Ni0.040 % the remainder iron, including typical elements accompanying steel that are not mentioned above, which represent contamination resulting from smelting.

Claims

1. A method, comprising: producing a steel strip from of a steel having a composition comprising the elements, in mass %: C0.0751 to 0.105 Si0.600 to 0.800 Mn1.000 to 1.900 Cr0.100 to 0.700 Al0.010 to 0.060 N 0.00200.0120 S0.0030 Nb0.005 to 0.050 Ti0.0050 to 0.050 B0.0005 to 0.0040 Mo0.200 Cu0.040% Ni0.040% remainder iron and steel accompanying elements constituting smelting related impurities, heating the steel strip during a continuous annealing to an annealing temperature in a range of 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 100 C./s from the first intermediate temperature to room temperature, wherein a sum of contents of Mn+Si+Cr of 2.40 and 2.70%, and the steel strip has a thickness of up to 1.00 mm.

2. 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.

3. The method of claim 1, wherein the steel 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.

4. The method of claim 1, wherein the heating step is performed using a plant configuration comprising a directly fired furnace and a radiant tube furnace, said method further comprising: 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*Si0.32,2*Mn0.450.1*Cr0.412.5*(InB)0.25, wherein Si, Mn, Cr and B are corresponding alloy proportions in the steel in mass % and pO.sub.2 is the oxygen partial pressure in mbar, and wherein a dew point of an overall atmosphere of the plant configuration to 30 C. or below for avoiding oxidation of the strip directly prior to immersion into a hot dip bath.

5. The method of claim 1, wherein the heating is performed with a single radiant tube furnace, and wherein the oxygen partial pressure of the atmosphere of the radiant tube furnace satisfies the following equation,
12>Log pO.sub.25*Si0.253*Mn050.1*Cr0.57*(InB)0.5 wherein Si, Mn, Cr, and B are corresponding alloy components in the steel in mass % and pO.sub.2 is an oxygen partial pressure in mbar, and wherein a dew point of an overall atmosphere of the plant configuration to 30 C. or below for avoiding oxidation of the strip directly prior to immersion into a hot dip bath.

6. The method of the 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.

7. The method of claim 1, further comprising after the heat treatment skin-passing the steel strip.

8. The method of claim 1, further comprising after the heat treatment stretch leveling the steel strip.

9. The method of claim 1, wherein the steel has a minimum tensile strength of 750 MPa and a yield to tensile ratio of maximally 73%.

10. The method of claim 1, wherein the Mn content is 1.500%.

11. A method, comprising: producing a steel strip from of a steel having a composition comprising the elements, in mass %: C0.075 to 0.105 Si0.600 to 0.800 Mn1.000 to 1.900 Cr0.100 to 0.700 Al0.010 to 0.060 N 0.00200.0120 S0.0030 Nb0.005 to 0.050 Ti0.0050 to 0.050 B0.0005 to 0.0040 Mo0.200 Cu0.040% Ni0.040% remainder iron and steel accompanying elements constituting smelting related impurities, heating the steel strip during a continuous annealing to an annealing temperature in a range of 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 100 C./s from the first intermediate temperature to room temperature, wherein a sum of contents of Mn+Si+Cr of 2.60 and 2.90%, and the steel strip has a thickness of 1.00-2.00 mm.

12. The method of claim 11, wherein the Mn content is 1.750%.

13. A method, comprising: producing a steel strip from of a steel having a composition comprising the elements, in mass %: C0.075 to 0.105 Si0.600 to 0.800 Mn1.000 to 1.900 Cr0.100 to 0.700 Al0.010 to 0.060 N 0.00200.0120 S0.0030 Nb0.005 to 0.050 Ti0.0050 to 0.050 B0.0005 to 0.0040 Mo0.200 Cu0.040% Ni0.040% remainder iron and steel accompanying elements constituting smelting related impurities, heating the steel strip during a continuous annealing to an annealing temperature in a range of 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 100 C./s from the first intermediate temperature to room temperature, wherein a sum of contents of Mn+Si+Cr of 2.80 and 3.10%, and the steel strip has a thickness of >2.00 mm.

14. The method of claim 13, wherein the Mn content is 1.500%.

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: a process chain (schematically) for the production of a strip from the steel according to the invention

(4) FIG. 2: a Time-temperature-course (schematically) of the process steps hot rolling and cold rolling (optionally) and continuous annealing, exemplary for the steel according to the invention

(5) FIG. 3: an example for analytical differences of the steel according to the invention relative to a carbon-rich (C0.120%) and micro-alloyed comparative grade

(6) FIG. 4: examples for mechanical characteristic values (Longitudinal to the rolling direction) of the steel according to the invention

(7) FIG. 5: Results of he hole expansion tests according to ISO 16630 (sheet thickness 1.00 mm and 2.00 mm) exemplary for the steel according to the invention relative to a carbon rich (C1.120%) and micro-alloyed comparative grade.

(8) FIGS. 6a, b, c: Temperature-time curves (annealing variants schematically).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(9) FIG. 1 schematically shows the process chain for the production of the steel according to the invention. Shown are the different process routes relating to the invention. Up to the hot rolling (final rolling temperature) the process route is the same for all steel according to the invention, thereafter different process routes are followed depending on the desired results. For example the pickled hot strip can be galvanized or can be cold rolled with different reduction degrees and galvanized. Or soft annealed hot strip or soft annealed cold strip can be cold rolled and galvanized. Also material can be optionally processed without zinc pot (continuous annealing) with and without subsequent electrolytic galvanization.

(10) FIG. 2 shows schematically the time-temperature-course of the process steps hot tolling and continuous annealing of strips having the alloy composition according to the invention. Shown are the time and temperature dependent transformation for the hot rolling process as well as for a heat treatment after the cold rolling.

(11) FIG. 3 exemplarily shows the essential alloy elements of the steel according to the invention, compared to the comparative grade. The steel according to the invention is significantly Si-alloyed. The comparative steel (standard grade) differs also regarding the carbon content, which is at 0.120%, but also regarding the elements titanium and boron.

(12) In addition the standard grade is niobium micro-alloyed like the steel according to the invention.

(13) FIG. 4 shows examples of mechanical characteristic values longitudinal relative to the rolling direction of the steel according to the invention.

(14) FIG. 5 shows results of the hole expansion tests according to ISO 16630 (absolute values and relative values to the comparative grade). Shown are the results of the hole expansion tests for variant A (coiling temperature above bainite start temperature) and variant B (coiling temperature below the bainite start temperature), respectively for process 2 and process 3.

(15) The materials have a sheet thickness of 1.00 mm or 2.000. The results apply for the test according to ISO 16630. It can be seen that the steels according to the invention have better or approximately same expansion values for punched holes as the comparative grades with same processing. The method 2 hereby corresponds to an annealing for example on a hot dip galvanizing with combined direct-fired furnace and radiant tube furnace as described in FIG. 6b. The method corresponds for example to a process control in a continuous annealing plant as described in FIG. 6c. In addition in this case a reheating of the steel can be achieved directly prior to the zinc bath by means of induction furnaces.

(16) The different temperature profiles according to the invention within the stated range result in characteristic values or different hole expansion results that are different from each other, that are significantly improved for the method 3 according to FIG. 6c compared to the comparative grades. A principle difference are also the temperature time parameters during the heat treatment and the following cooling.

(17) The FIGS. 6 schematically show three variants of the temperature time courses according to the invention at the annealing treatment and cooling and respectively different austenitization conditions.

(18) The method 1 (FIG. 6a) shows the annealing and cooling of produced cold or hot rolled or cold rerolled steel strip in a continuous annealing facility. First the strips is heated to a temperature in the range of about 700 to 950 C. The annealed steel strip is then cooled from the annealing temperature to an intermediate temperate 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. Afterwards this the steel strip is cooled at room temperature with a cooling rate between about 2 and 30 C./s until reaching room temperature or the cooling is maintained at a cooling rate of about 15 and 100 C./s until reaching room temperature.

(19) The method 2 (FIG. 6b) shows the process according to method 1, however the cooling of the steel strip for the purpose of the hot dip galvanization is briefly interrupted during passage through the hot dip galvanizing container, in order to then continue the cooling with a cooling rate of between about 15 and 100 C./s until reaching an intermediate temperature of about 200 to 250 C. The steel strip is then cooled at air with a cooling rate of between about 2 and 30 C./s until reaching room temperature.

(20) The method 3 (FIG. 6c) also shows the process according to method 1 in case of a hot dip coating, however, the cooling of the steel strip is interrupted by a brief brake (about 1 to 20s) at an intermediate temperature in the range of about 200 to 400 C. and is reheated to the temperature which is required for the hot dip coating (about 400 to 470 C.). Subsequent thereto the steel strip is again heated to an intermediate temperature of about 200 to 250 C. The final cooling of the steel strip at air to room temperature is conducted with a cooling rate of about 2 and 30 C./s.

(21) For the industrial production for the hot dip galvanizing according to method 2 according to FIG. 6b and according to method 3 according to FIG. 6c the following examples are given:

EXAMPLE 1 (Cold Rerolled Hot Strip)

(22) Variant B/2.00 mm/method 2 according to FIG. 6b

(23) A steel according to the invention with 0.091% C; 0.705% Si; 1.801% Mn; 0.010% P; 0.0030% S; 0.0054% N; 0.035 Al; 0.344% Cr; 0.012% Mo; 0.016% Ti; 0.001% V; 0.016% Nb; 0.0031% B was melted in a high vacuum melting and casting facility, hot rolled in a hot rolling stand at a final rolling target temperature of 910 C. and transported into the furnace at a reel temperature of 500 C. with a thickness of 2.30 mm for a simulated reel cooling. After sand blasting the cold rolling was conducted with a cold rolling degree of 15% from 2.30 to 2.00 mm.

(24) In an annealing simulator the steel was processed analogously to the hot dip galvanizing facility according to FIG. 6b.

(25) The steel according to the invention after the heat treatment has a microstructure which consists of ferrite, martensite, bainite and residual austenite.

(26) This steel has the following characteristic values:

(27) TABLE-US-00001 yield strength (Rp0.2) 461 MPa tensile strength (Rm) 821 MPa elongation at break (A80) 15.4% bake-hardening-index (BH2) 48 MPa hole expansion ratio according to ISO 16630 36%
longitudinal to the rolling direction and corresponds for example to a CR440y780T-DP according to VDA 239-100.

(28) The yield to tensile ratio Re/Rm in longitudinal direction is 56%.

EXAMPLE 2 (Cold Rerolled Strip)

(29) Variant B/2.00 mm/method 3 according to FIG. 6c

(30) A steel according to the invention with 0.091% C; 0.705% Si; 1.801% Mn; 0.010% P; 0.0030% S; 0.0054% N; 0.035 Al; 0.344% Cr; 0.012% Mo; 0.016% Ti; 0.001% V; 0.016% Nb; 0.0031% B was melted in a high vacuum melting and casting plant, hot rolled in a hot rolling stand at a final rolling target temperature of 910 C. and inserted in the furnace at a reel target temperature of 500 C. with a thickness of 2.30 mm for a simulated reel cooling. After the sand blasting the cold rolling was conducted with a cold rolling degree of 15% from 2.30 to 2.00 mm.

(31) In an annealing simulator the steel was processed analogous to a hot dip galvanizing plant according to FIG. 6c.

(32) After the heat treatment the steel according to the invention has a microstructure which consists of ferrite, martensite and residual austenite.

(33) This steel has the following characteristic values:

(34) TABLE-US-00002 yield strength (Rp0.2) 611 MPa tensile strength (Rm) 847 MPa elongation at break (A80) 10.2% bake-hardening-index (BH2) 52 MPa hole expansion ratio according to ISO 16630 41%
longitudinal to the rolling direction and corresponds for example to a CR570y780T-CP according to VDA 239-100. The yield to tensile ratio Re/Rm in longitudinal direction is 72%.

EXAMPLE 3 (Cold Strip)

(35) Variant A/1.00 mm/method 2 according to FIG. 6b

(36) A steel according to the invention with 0.091% C; 0.705% Si; 1.801% Mn; 0.010% P; 0.0030% S; 0.0054% N; 0.035 Al; 0.344% Cr; 0.012% Mo; 0.016% Ti; 0.001% V; 0.016% Nb; 0.0031% B was melted in a high vacuum melting and casting plant, hot rolled in a hot rolling stand at a final rolling target temperature of 910 C. and inserted in the furnace at a reel target temperature of 710 C. with a thickness of 2.02 mm for a simulated reel cooling. After the sand blasting the cold rolling was conducted with a cold rolling degree of 50% from 2.30 to 2.00 mm.

(37) In an annealing simulator the steel was processed analogous to a hot dip galvanizing plant according to FIG. 6b.

(38) After the heat treatment the steel according to the invention has a microstructure which consists of ferrite, martensite and residual austenite.

(39) This steel has the following characteristic values:

(40) TABLE-US-00003 yield strength (Rp0.2) 442 MPa tensile strength (Rm) 793 MPa elongation at break (A80) 14.5% bake-hardening-index (BH2) 51 MPa hole expansion ratio according to ISO 16630 48%
longitudinal to the rolling direction and corresponds for example to a CR570y780T-CP according to VDA 239-100. The yield tensile ratio Re/Rm in longitudinal direction is 56%.

EXAMPLE 4 (Cold Strip)

(41) Variant A/1.00 mm/method 3 according to FIG. 6c

(42) A steel according to the invention with 0.091% C; 0.705% Si; 1.801% Mn; 0.010% P; 0.0030% S; 0.0054% N; 0.035 Al; 0.344% Cr; 0.012% Mo; 0.016% Ti; 0.001% V; 0.016% Nb; 0,0031% B was melted in a high vacuum melting and casting plant, hot rolled in a hot rolling stand at a final rolling target temperature of 910 C. and inserted in the furnace at a reel target temperature of 710 C. with a thickness of 2.02 mm for a simulated reel cooling. After the sand blasting the cold rolling was conducted with a cold roiling degree of 50% from 2.02 to 0.99 mm.

(43) In an annealing simulator the steel was processed analogous to a hot dip galvanizing plant according to FIG. 6c.

(44) After the heat treatment the steel according to the invention has a microstructure which consists of ferrite, martensite and residual austenite.

(45) This steel has the following characteristic values:

(46) TABLE-US-00004 yield strength (Rp0.2) 520 MPa tensile strength (Rm) 780 MPa elongation at break (A80) 14.2% bake-hardening-index (BH2) 46 MPa hole expansion ratio according to ISO 16630 67%
longitudinal to the rolling direction and corresponds for example to a CR570y780T-CP according to VDA 239-100. The yield to tensile ratio Re/Rm in longitudinal direction is 67%.

EXAMPLE 5 (Hot Strip)

(47) Variant A/2.00 mm/method 2 according to FIG. 6b

(48) A steel according to the invention with 0.091% C; 0.705% Si; 1.801% Mn; 0.010% P; 0.0030% S; 0.0054% N; 0.035 Al; 0.344% Cr; 0.012% Mo; 0.016% Ti; 0.001% V; 0.016% Nb; 0.0031% B was melted in a high vacuum melting and casting plant, hot rolled in a hot rolling stand at a final rolling target temperature of 910 C. and inserted in the furnace at a reel target temperature of 710 C. with a thickness of 2.02 mm for a simulated reel cooling. After the sand blasting the annealing was conducted.

(49) In an annealing simulator the steel was processed analogous to a hot dip galvanizing plant according to FIG. 6b.

(50) After the heat treatment the steel according to the invention has a microstructure which consists of ferrite, martensite and residual austenite.

(51) This steel has the following characteristic values:

(52) TABLE-US-00005 yield strength (Rp0.2) 580 MPa tensile strength (Rm) 844 MPa elongation at break (A80) 10.9% bake-hardening-index (BH2) 47 MPa hole expansion ratio according to ISO 16630 45%
longitudinal to the rolling direction and corresponds for example to a CR570y780T-CP according to VDA 239-100. The yield to tensile ratio Re/Rm in longitudinal direction is 69%.

EXAMPLE 6 (Hot Strip)

(53) Variant A/2.00 mm/method 3 according to FIG. 6c

(54) A steel according to the invention with 0.091% C; 0.705% Si; 1.801% Mn; 0.010% P; 0.0030% S; 0.0054% N; 0.035 Al; 0.344% Cr; 0.012% Mo; 0.016% Ti; 0.001% V; 0.016% Nb; 0.0031% B was melted in a high vacuum melting and casting plant, hot rolled in a hot rolling stand at a final rolling target temperature of 910 C. and inserted in the furnace at a reel target temperature of 710 C. with a thickness of 2.02 mm for a simulated reel cooling. After sand blasting the annealing was conducted.

(55) In an annealing simulator the steel was processed analogous to a hot dip galvanizing plant according to FIG. 6c.

(56) After the heat treatment the steel according to the invention has a microstructure which consists of ferrite, martensite and residual austenite.

(57) This steel has the following characteristic values:

(58) TABLE-US-00006 yield strength (Rp0.2) 661 MPa tensile strength (Rm) 908 MPa elongation at break (A80) 10.1% bake-hardening-index (BH2) 51 MPa hole expansion ratio according to ISO 16630 77%
longitudinal to the rolling direction and corresponds for example to a CR570y780T-CP according to VDA 239-100. The yield to tensile ratio Re/Rm in longitudinal direction is 72%.

EXAMPLE 7 (Hot Strip)

(59) Variant A/2.30 mm/method 2 according to FIG. 6b.

(60) A steel according to the invention with 0.091% C; 0.705% Si; 1.801% Mn; 0.010% P; 0.0030% S; 0.0054% N; 0.035 Al; 0.344% Cr; 0.012% Mo; 0.016% Ti; 0.001% V; 0.016% Nb; 0.0031% B was melted in a high vacuum melting and casting plant, hot rolled in a hot rolling stand at a final rolling target temperature of 910 C. and inserted in the furnace at a coil target temperature of 500 C. with a thickness of 2.30 mm for a simulated coil cooling. After the sand blasting the annealing was conducted.

(61) In an annealing simulator the steel was processed analogous to a hot dip galvanizing plant according to FIG. 6b.

(62) After the heat treatment the steel according to the invention has a microstructure which consists of ferrite, martensite and residual austenite.

(63) This steel has the following characteristic values:

(64) TABLE-US-00007 yield strength (Rp0.2) 565 MPa tensile strength (Rm) 830 MPa elongation at break (A80) 10.7% bake-hardening-index (BH2) 53 MPa hole expansion ratio according to ISO 16630 42%
longitudinal to the rolling direction and corresponds for example to a CR570y780T-CP according to VDA 239-100. The yield to tensile ratio Re/Rm in longitudinal direction is 68%.

EXAMPLE 8 (Hot Strip)

(65) Variant B/2.30 mm/method 3 according to FIG. 6c.

(66) A steel according to the invention with 0.091% C; 0.705% Si; 1.801% Mn; 0.010% P; 0.0030% S; 0.0054% N; 0.035 Al; 0.344% Cr; 0.012% Mo; 0.016% Ti; 0.001% V; 0.016% Nb; 0.0031% B was melted in a high vacuum melting and casting plant, hot rolled in a hot rolling stand at a final rolling target temperature of 910 C. and inserted in the furnace at a reel target temperature of 500 C. with a thickness of 2.30 mm for a simulated reel cooling. After the sand blasting the annealing was conducted.

(67) In an annealing simulator the steel was processed analogous to a hot dip galvanizing plant according to FIG. 6c.

(68) After the heat treatment the steel according to the invention has a microstructure which consists of ferrite, martensite and residual austenite.

(69) This steel has the following characteristic values:

(70) TABLE-US-00008 yield strength (Rp0.2) 661 MPa tensile strength (Rm) 905 MPa elongation at break (A80) 10.6% bake-hardening-index (BH2) 49 MPa hole expansion ratio according to ISO 16630 54%
longitudinal to the rolling direction and corresponds for example to a CR570y780T-CP according to VDA 239-100. The yield to tensile ratio Re/Rm in longitudinal direction is 73%.

(71) FIG. 1: process chain (schematic) for the production of a strip made of the steel according to the invention. 1. furnace process 2. secondary metallurgy 3. continuous casting 4. hot rolling 5. pickling 6. soft annealing hot strip (optional) 7. cold rolling (optional_ 8. dual roller (optional) 9. soft annealing cold strip (optional) 10. hot dip galvanizing/continuous annealing 11. inline skin-passing 12.stretch leveling