Steel for press hardening and press hardened part manufactured from such steel

Patent metadata

Field	Value
Publication number	US-11319610-B2
Application number	US-201615741290-A
Country	US
Kind code	B2
Filing date	Jun 10, 2016
Priority date	Jul 9, 2015
Publication date	May 3, 2022
Grant date	May 3, 2022

Abstract

A process for manufacturing a press hardened steel part is provided. The steel of the part has a chemical composition including, in weight: 0.062%≤C≤0.095%, 1.4%≤Mn≤1.9%, 0.2%≤Si≤0.5%, 0.020%≤Al≤0.070%, 0.02%≤Cr≤0.1%, wherein: 1.5%≤(C+Mn+Si+Cr)≤2.7%, 0.040%≤Nb≤0.060%, 3.4×N≤Ti≤8×N wherein: 0.044%≤(Nb+Ti)≤0.090%, 0.0005≤B≤0.004%, 0.001%≤N≤0.009%, 0.0005%≤S≤0.003%, 0.001%≤P≤0.020%, optionally: 0.0001%≤Ca≤0.003%, and the remainder being Fe and unavoidable impurities. The process includes hot forming the heated blank in the forming press so as to obtain a formed part and cooling the formed part at a cooling rate CR1 between 40 and 360°C/s in a temperature range from 750 to 450°C. and at a cooling rate CR2 between 15 to 150°C/s in a temperature range from 450°C to 250°C. wherein CR2<CR1.

First claim

What is claimed is:

1. A process for manufacturing a press hardened steel part comprising the successive steps of:
providing a steel semi-product with a composition including, by weight:
0.062≤C≤0.095%;

1.4%≤Mn≤1.9%;

0.2%≤Si≤0.5%;

0.020%≤Al≤0.070%;

0.02%≤Cr≤0.1%;

wherein: 1.5%≤(C+Mn+Si+Cr)≤2.7%;

0.040%≤Nb≤0.060%;

3.4×N≤Ti≤8×N;

wherein: 0.044%≤(Nb+Ti)≤0.090%;

0.0005≤B≤0.004%;

0.001%≤N≤0.009%;

0.0005%≤S≤0.003%;

0.001%≤P≤0.020%; and

a remainder being Fe and unavoidable impurities;
hot rolling the steel semi-product to obtain a hot rolled steel sheet;
coiling the hot rolled steel sheet at a coiling temperature Tc comprised between 550° C. and Ms, Ms being a martensitic transformation start temperature of the steel sheet, so as to obtain a coiled steel sheet;
optionally cold rolling the coiled steel sheet;
annealing the steel sheet at an annealing temperature Ta in order to obtain less than 10% of unrecrystallized area fraction, so as to obtain an annealed steel sheet;
cutting the annealed steel sheet to a predetermined shape, so as to obtain a blank;
heating the blank with a heating device and holding the blank at a temperature Tm between 890 and 950° C., a holding duration Dm at the temperature being between 1 and 10 minutes, so as to obtain a heated blank;
extracting said heated blank from the heating device and transferring said heated blank within a forming press, the transfer duration Dt being less than 10 s;
hot forming the heated blank in the forming press so as to obtain a formed part;
cooling the formed part at a first cooling rate CR1 between 40 and 360° C/s in a temperature range between 750 and 450° C., and then at a second cooling rate CR2 between 15 to 150° C/s in a temperature range between 450° C. and 250° C., wherein CR2 <CR1.

2. The process for manufacturing a press hardened steel part according to claim 1 , wherein the coiled steel sheet is cold rolled with a rolling ratio between 50 and 80%.

3. The process for manufacturing a press hardened steel part according to claim 1 , wherein the annealing temperature Ta is between 800 and 850° C.

4. The process for manufacturing a press hardened steel part according to claim 1 , wherein the annealing temperature Ta is between 800 and 835° C.

5. The process for manufacturing a press hardened steel part according to claim 1 , wherein the blank is cold formed before heating the blank at the temperature Tm.

6. The process for manufacturing a press hardened steel part according to claim 1 , wherein the hot forming is performed with a deformation quantity ε c higher than 0.15 in at least one hot deformed zone of the part.

7. The process for manufacturing a press hardened steel part according to claim 1 , wherein the annealed steel sheet is precoated with a metallic precoating before cutting the annealed steel blank to a predetermined shape.

8. The process for manufacturing a press hardened steel part according to claim 1 , wherein the holding duration Dm is between 1 and 6 minutes.

9. The process for manufacturing a press hardened steel part according to claim 1 , wherein modes and quantities of deformation during the hot forming differ from one zone to another of the heated blank.

10. The process for manufacturing a press hardened steel part according to claim 7 , wherein the metallic precoating is zinc, or zinc-based alloy, or zinc alloy.

11. The process for manufacturing a press hardened steel part according to claim 7 , wherein the metallic precoating is aluminum, or aluminum-based alloy, or aluminum alloy.

12. The process for manufacturing a press hardened steel part according to claim 7 , wherein the metallic precoating comprises at least one intermetallic layer containing A1 and iron, and optionally silicon, and wherein the metallic precoating contains neither free A1, nor τ 5 phase of Fe 3 Si 2 Al 12 type, nor τ 6 phase of Fe 2 Si 2 Al 9 .

13. The process for manufacturing a press hardened steel part according to claim 7 , wherein the metallic precoating comprises a layer of aluminum or an aluminum-based alloy or an aluminum alloy, topped by a layer of zinc or zinc-based alloy or a zinc alloy.

14. The process for manufacturing a press hardened steel part according to claim 9 , comprising deforming zones of the heated blank with an equivalent strain ε b higher than 0.15.

15. The process for manufacturing a press hardened steel part according to claim 14 , wherein an average bainitic/martensitic lath size width in the zones deformed at the equivalent strain ε b is less than 1 μm.

16. The process for manufacturing a press hardened steel part according to claim 15 , further comprising deforming a zone of the heated blank at a strain ε b less than 0.05.

17. The process for manufacturing a press hardened steel part according to claim 16 , wherein an average bainitic/martensitic lath size width of the zone deformed at the strain ε b less than 0.05 is between 1 and 2.5 μm.

Assignees

• Arcelormittal

Inventors

• Drillet Pascal
• Poirier Maria
• Sarkar Sujay

Classifications

• C21D8/02 Primary

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C22C38/38

  with more than 1.5% by weight of manganese · CPC title

• C21D8/0263

  following hot rolling · CPC title

• C21D8/0236

  Cold rolling · CPC title

• B21B1/22

  for rolling {plates, strips,} bands or sheets of indefinite length (B21B1/42 takes precedence) · CPC title