HIGH FORMABILITY STEEL SHEET FOR THE MANUFACTURE OF LIGHTWEIGHT STRUCTURAL PARTS AND MANUFACTURING PROCESS

• US 20200131607A1
• Filed: 04/20/2018
• Published: 04/30/2020
• Est. Priority Date: 04/21/2017
• Status: Active Grant

• Current Assignee: Arcelormittal (ArcelorMittal SA)
• Original Assignee: Arcelormittal (ArcelorMittal SA)
• Inventors: BONNET, Frederic, BOBADILLA, Manuel, BELE, Bertrand, DAESCHLER, Valerie
• Granted Patent: US 11,427,898 B2
• Time in Patent Office: Days
• CPC Class Codes: B22D 11/001 of specific alloys
• CPC Class Codes: B22D 11/0622 formed by two casting wheels
• CPC Class Codes: B22D 11/1206 for plastic shaping of strands rolling mills B21B1/46
• CPC Class Codes: B22D 11/225 for secondary cooling
• CPC Class Codes: C21D 2211/001 Austenite
• CPC Class Codes: C21D 2211/004 Dispersions; Precipitations
• CPC Class Codes: C21D 2211/005 Ferrite
• CPC Class Codes: C21D 8/0215 Rapid solidification; Thin strip casting
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C21D 8/0236 Cold rolling
• 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 38/08 containing nickel C22C38/105 takes precedence
• CPC Class Codes: C22C 38/12 containing tungsten, tantalum, molybdenum, vanadium, or niobium
• CPC Class Codes: C22C 38/14 containing titanium or zirconium
• CPC Class Codes: C22C 38/28 with titanium or zirconium
• CPC Class Codes: C22C 38/32 with boron

Abstract

A steel sheet has a composition comprising, by weight: 0.010%≤C≤0.080%, 0.06%≤Mn≤3%, Si≤1.5%, 0.005%≤Al≤1.5%, S≤0.030%, P≤0.040%, Ti and B such that: 3.2%≤Ti≤7.5% and (0.45×Ti)−1.35≤B≤(0.45×Ti)−0.43, optionally Ni≤1%, Mo≤1%, Cr≤3%, Nb≤0.1%, V≤0.1%, the remainder being iron and unavoidable impurities resulting from the smelting. The steel sheet has a structure consisting of ferrite, at most 10% of austenite, and precipitates comprising eutectic precipitates of TiB₂, the volume fraction of TiB₂ precipitates with respect to the whole structure being of at least 9%, the proportion of TiB₂ precipitates having a surface area lower than 8 μm² being of at least 96%.

Claims

1-27. -27. (canceled)

28. A steel sheet made of a steel having a composition comprising, by weight percent:

0.010%≤

C≤

0.080%

0.06%≤

Mn≤

3%

Si≤

1.5%

0.005%≤

Al≤

1.5%

S≤

0.030%

P≤

0.040%,

Ti and B such that;

3.2%≤

Ti≤

7.5%

(0.45×

Ti)−

1.35≤

B≤

(0.45×

Ti)−

0.43

optionally one or more elements chosen amongst;

Ni≤

1%

Mo≤

1%

Cr≤

3%

Nb≤

0.1%

V≤

0.1%

a remainder being iron and unavoidable impurities resulting from smelting,

the steel sheet having a structure consisting of ferrite, at most 10% of austenite, and precipitates, the precipitates comprising eutectic precipitates of TiB₂, a volume fraction of TiB₂ precipitates with respect to a whole structure being of at least 9%, a proportion of TiB₂ precipitates having a surface area lower than 8 μ

m² being of at least 96%.

29. The steel sheet according to claim 28, wherein a proportion of TiB₂ precipitates having a surface area lower than 3 μ

m² is of at least 80%.

30. The steel sheet according to claim 28, wherein a proportion of TiB₂ precipitates having a surface area lower than 25 μ

m² is of 100%.

31. The steel sheet according to claim 28, wherein in a core region of the steel sheet, a proportion of TiB₂ precipitates having a surface area lower than 8 μ

m² is of at least 96%.

32. The steel sheet according to claim 31, wherein in the core region of the steel sheet, a proportion of TiB₂ precipitates having a surface area lower than 3 μ

m² is of at least 80%.

33. The steel sheet according to claim 31, wherein in the core region of the steel sheet, a proportion of TiB₂ precipitates having a surface area lower than 25 μ

m² is of 100%.

34. The steel sheet according to claim 28, wherein the steel sheet comprises no TiC precipitates, or comprises TiC precipitates with a volume fraction lower than 0.5%.

35. The steel sheet according to claim 28, wherein the steel sheet comprises no Fe₂B precipitates.

36. The steel sheet according to claim 28, wherein the titanium, boron and manganese contents are such that:

(0.45×

Ti)−

1.35≤

B≤

(0.45×

Ti)−

(0.261*Mn)−

0.414.

37. The steel sheet according to claim 28, wherein the titanium and boron contents are such that:

(0.45×

Ti)−

1.35≤

B≤

(0.45×

Ti)−

0.50.

38. The steel sheet according to claim 28, wherein the composition is such that C≤

0.050%.

39. The steel sheet according to claim 28, wherein the composition is such that Al≤

1.3%.

40. The steel sheet according to claim 28, wherein the steel sheet has a Charpy energy Kcv of at least 25 J/cm² at −

40°

C.

41. The steel sheet according to claim 28, wherein the steel sheet has a content in free Ti of at least 0.95%.

42. A method for manufacturing a structural part, the method comprising:

cutting at least one blank from the steel sheet according to claim 28, and

deforming the blank within a temperature range from 20°

C. to 900°

C.

43. The method according to claim 42, comprising, before deforming the blank, a step of welding the blank to another blank.

44. A process for manufacturing a steel sheet, the process comprising the following successive steps:

providing a steel having a composition comprising, by weight percent;

0.010%≤

C≤

0.080%

0.06%≤

Mn≤

3%

Si≤

1.5%

0.005%≤

Al≤

1.5%

S≤

0.030%

P≤

0.040%,

Ti and B such that;

3.2%≤

Ti≤

7.5%

(0.45×

Ti)−

1.35≤

B≤

(0.45×

Ti)−

0.43

optionally one or more elements chosen amongst;

Ni≤

1%

Mo≤

1%

Cr≤

3%

Nb≤

0.1%

V≤

0.1%

a remainder being iron and unavoidable impurities,

casting the steel to form a semi-product, with a casting temperature lower than or equal to L_liquidus+40°

C., L_liquidus designating a liquidus temperature of the steel, the semi-product being cast in a form of a thin semi-product having a thickness of at most 110 mm, the steel being solidified during the casting with a solidification rate comprised between 0.03 cm/s and 5 cm/s at every location of the semi-product.

45. The process according to claim 44, wherein the semi-product is cast in a form of a thin slab having a thickness lower than or equal to 110 mm.

46. The process according to claim 45, wherein the semi-product is cast in a form of a thin slab having a thickness lower than or equal to 70 mm.

47. The process according to claim 45, wherein the semi-product is cast by compact strip production.

48. The process according to claim 44, wherein the semi-product is cast in a form of a thin strip having a thickness lower than or equal to 6 mm, the solidification rate being comprised between 0.2 cm/s and 5 cm/s at every location of the semi-product.

49. The process according to claim 48, wherein the semi-product is cast by direct strip casting between counter-rotating rolls.

50. The process according to claim 44, wherein, after the casting and the solidification, the semi-product is hot rolled, to obtain a hot-rolled steel sheet.

51. The process according to claim 50, wherein between the casting and the hot-rolling, a temperature of the semi-product remains higher than 700°

C.

52. The process according to claim 50, wherein before the hot-rolling, the semi-product is de-scaled at a temperature of at least 1050°

C.

53. The process according to claim 50, wherein after the hot-rolling, the hot-rolled steel sheet is cold-rolled, to obtain a cold-rolled steel sheet having a thickness lower than or equal to 2 mm.

54. The process according to claim 44, wherein the titanium, boron and manganese contents are such that:

(0.45×

Ti)−

1.35≤

B≤

(0.45×

Ti)−

(0.261*Mn)−

0.414.

55. The process according to claim 44, wherein the composition is such that Al <

1.3%.

56. A structural part comprising at least a portion made of a steel having a composition comprising, by weight percent:

0.010%≤

C≤

0.080%

0.06%≤

Mn≤

3%

Si≤

1.5%

0.005%≤

Al≤

1.5%

S≤

0.030%

P≤

0.040%,

Ti and B such that;

3.2%≤

Ti≤

7.5%

(0.45×

Ti)−

1.35≤

B≤

(0.45×

Ti)−

0.43

optionally one or more elements chosen amongst;

Ni≤

1%

Mo≤

1%

Cr≤

3%

Nb≤

0.1%

V≤

0.1%

a remainder being iron and unavoidable impurities resulting from smelting,

the portion having a structure consisting of ferrite, at most 10% of austenite, and precipitates, the precipitates comprising eutectic precipitates of TiB₂, a volume fraction of TiB₂ precipitates with respect to a whole structure of the portion being of at least 9%, a proportion of TiB₂ precipitates having a surface area lower than 8 μ

m² being of at least 96%.