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

• US 10,640,855 B2
• Filed: 11/04/2015
• Issued: 05/05/2020
• Est. Priority Date: 11/18/2014
• Status: Active Grant

• Current Assignee: Salzgitter Flachstahl GmbH (Salzgitter AG)
• Original Assignee: Salzgitter Flachstahl GmbH (Salzgitter AG)
• Inventors: Schulz, Thomas, Schottler, Joachim, Kluge, Sascha, Meyer, Christian, Matthies, Peter, Wedemeier, Andreas
• Primary Examiner(s): Wu, Jenny R
• Application Number:
  US15/527,794
• Publication Number:
  US 20180347018A1
• Time in Patent Office: 1,644 Days
• CPC Class Codes: C21D 1/26 Methods of annealing
• CPC Class Codes: C21D 1/76 Adjusting the composition of the atmosphere
• CPC Class Codes: C21D 1/84 Controlled slow cooling cooling-beds for metal rolling B21B43/00
• CPC Class Codes: C21D 2211/002 Bainite
• CPC Class Codes: C21D 2211/005 Ferrite
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 6/002 containing Cr C21D6/004 takes precedence
• CPC Class Codes: C21D 6/005 containing Mn
• CPC Class Codes: C21D 6/007 containing Co
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C21D 8/0236 Cold rolling
• CPC Class Codes: C21D 8/0247 characterised by the heat treatment
• CPC Class Codes: C21D 8/0273 Final recrystallisation annealing
• CPC Class Codes: C21D 8/0447 characterised by the heat treatment
• CPC Class Codes: C21D 8/0473 Final recrystallisation annealing
• CPC Class Codes: C21D 9/46 for sheet metals
• CPC Class Codes: C21D 9/48 deep-drawing sheets
• CPC Class Codes: C21D 9/561 with a controlled atmosphere or vacuum
• CPC Class Codes: C21D 9/562 Details
• CPC Class Codes: C21D 9/58 with heating by baths
• CPC Class Codes: C22C 38/001 : containing N
• CPC Class Codes: C22C 38/002 : containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• CPC Class Codes: C22C 38/02 : containing silicon
• CPC Class Codes: C22C 38/04 : containing manganese
• CPC Class Codes: C22C 38/06 : containing aluminium
• CPC Class Codes: C22C 38/26 : with niobium or tantalum
• CPC Class Codes: C22C 38/28 : with titanium or zirconium
• CPC Class Codes: C22C 38/32 : with boron
• CPC Class Codes: C22C 38/38 : with more than 1.5% by weight of manganese
• CPC Class Codes: C22C 38/58 : with more than 1.5% by weight of manganese
• CPC Class Codes: C23C 2/0038 : characterised by the pre-treatment chambers located immediately upstream of the bath or occurring locally before the dipping process
• CPC Class Codes: C23C 2/02 : Pretreatment of the material to be coated, e.g. for coating on selected surface areas C23C2/30 takes precedence
• CPC Class Codes: C23C 2/0224 : Two or more thermal pretreatments
• CPC Class Codes: C23C 2/06 : Zinc or cadmium or alloys based thereon
• CPC Class Codes: C23C 2/26 : After-treatment C23C2/14 takes precedence
• CPC Class Codes: C23C 2/40 : Plates; Strips

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: C≥0.075 to ≤0.115; Si≥0.200 to ≤0.300; Mn≥1.700 to ≤2.300; Cr≥0.280 to ≤0.4800; Al≥0.020 to ≤0.060; N≥0.0020 to ≤0.0120; S≤0.0050; Nb≥0.005 to ≤0.050; Ti≥0.005 to ≤0.050; B≥0.0005 to ≤0.0060; Ca≥0.0005 to ≤0.0060; Cu≤0.050; Ni≤0.050; 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;

C≥

0.075 to ≤

0.115

Si≥

0.200 to ≤

0.300

Mn≥

1.700 to ≤

2.300

Cr≥

0.280 to ≤

0.4800

Al≥

0.020 to ≤

0.060

N≥

0.0020 to ≤

0.0120

S≤

0.0050

Nb≥

0.005 to ≤

0.050

Ti≥

0.005 to ≤

0.050

B≥

0.0005 to ≤

0.0060

Ca≥

0.0005 to ≤

0.0060

Cu≤

0.050

Ni≤

0.050,

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 pO₂≥

5*Si<sup>−

0.3</sup>−

2,2*Mn<sup>−

0.45</sup>−

0.1*Cr<sup>−

0.4</sup>−

12.5*(−

InB)^0.25,

wherein Si, Mn, Cr and B are corresponding alloy proportions in the steel in % by weight and pO₂ 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 30°

C./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;

C≥

0.075 to ≤

0.115

Si≥

0.200 to ≤

0.300

Mn≥

1.700 to ≤

2.300

Cr≥

0.280 to ≤

0.4800

Al≥

0.020 to ≤

0.060

N≥

0.0020 to ≤

0.0120

S≤

0,0050

Nb≥

0.005 to ≤

0.050

Ti≥

0.005 to ≤

0.050

B≥

0.0005 to ≤

0.0060

Ca≥

0.0005 to ≤

0.0060

Cu≤

0.050

Ni≤

0.050,

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 pO₂≥

5*Si<sup>−

0.25</sup>−

3*Mn<sup>−

0.5</sup>−

0.1*Cr<sup>−

0.5</sup>−

7*(−

InB)^0.5

wherein Si, Mn, Cr, and B are corresponding alloy components in the steel in % by weight and pO₂ 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.