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JP-2026514392-A - High-strength cold-rolled and annealed steel and method for manufacturing the same

JP2026514392AJP 2026514392 AJP2026514392 AJP 2026514392AJP-2026514392-A

Abstract

The present invention relates to high-strength cold-rolled annealed steel and a method (100) for producing high-strength cold-rolled annealed steel. The high-strength cold-rolled annealed steel has the following composition expressed in weight percent: C: 0.25 to 0.35, Mn: 1.0 to 2.5, S: < 0.006, P: < 0.02, Si: 0.02 to 0.50, Al: 0.02 to 0.50, Mo: 0.00 to 0.30, N (ppm) < 80, with the remainder being iron (Fe) and unavoidable impurities. The high-strength cold-rolled annealed steel contains a microstructure of 10 to 30% bainite and 70 to 90% martensite. The high-strength cold-rolled annealed steel exhibits a tensile strength in the range of 1400 to 1800 MPa and a minimum elongation of 7%.

Inventors

  • ランジャン、ラヴィ
  • ゴーシュ、チラディープ
  • バッタチャルヤ、バスデブ
  • サイード、バディルジャマン
  • チャッタージー、スーラブ
  • ダッタ、モノジット

Assignees

  • タタ スチール リミテッド

Dates

Publication Date
20260511
Application Date
20240326
Priority Date
20230327

Claims (12)

  1. A high-strength cold-rolled annealed steel having the following composition expressed in weight percent: Carbon (C): 0.25-0.35%, Manganese (Mn): 1.0% to 2.0% Silicon (Si): 0.02% to 0.5% Sulfur (S): <0.006% Phosphorus (P): <0.02% Aluminum (Al): 0.02-0.5%, Molybdenum (Mo): <0.3% High-strength cold-rolled and annealed steel containing nitrogen (N) < 80 ppm, with the remainder being substantially iron and associated impurities, and containing a microstructure of 10-30% bainite and 70-90% martensite.
  2. The high-strength cold-rolled and annealed steel according to claim 1, exhibiting an ultimate tensile strength (UTS) in the range of 1400 to 1800 MPa, a yield strength (YS) of 1000 to 1300 MPa, a minimum uniform elongation of 4%, and a minimum total elongation of 7%.
  3. The high-strength cold-rolled annealed steel according to claim 1, wherein the carbon content in the high-strength cold-rolled annealed steel is optimally maintained to improve the hardenability and hardness of the martensite.
  4. The high-strength cold-rolled annealed steel according to claim 1, wherein the Mn content in the high-strength cold-rolled annealed steel further improves hardenability and delays ferrite transformation.
  5. The high-strength cold-rolled annealed steel according to claim 1, wherein the Mo content improves the hardenability of the steel and increases the rate of bainite formation during continuous annealing.
  6. A method (100) for producing a high-strength cold-rolled annealed steel sheet or high-strength cold-rolled annealed steel strip having a thickness in the range of 0.5 mm to 2 mm, A process of casting molten steel to obtain a steel slab, having a composition expressed in weight percent: C: 0.25-0.35, Mn: 1.0-2.5, S: <0.006, P: <0.02, Si: 0.02-0.50, Al: 0.02-0.50, Mo: 0.00-0.30, N (ppm) < 80, with the remainder being iron (Fe) and unavoidable impurities; The process involves reheating the steel slab to a temperature in the range of 1200 to 1280°C; A process of roughly processing a steel slab in a rough rolling mill having an outlet temperature in the range of 1000 to 1080°C; A process for manufacturing steel sheets by hot-rolling a roughly-rolled steel slab, with finish rolling performed at temperatures ranging from 830°C to 880°C ( TFRT ); A step of obtaining first intermediate hot-rolled steel by cooling hot-rolled steel to a first intermediate temperature in the range of 640 to 680°C at a first intermediate cooling rate in the range of 0.5°C/sec to 50°C/sec; The first step involves winding the intermediate hot-rolled steel at a winding temperature in the range of 640 to 680°C; The first step involves pickling the intermediate hot-rolled steel to remove scale from the surface of the steel strip; A process of further thinning a steel strip by cold rolling it to obtain cold-rolled steel having a predetermined thickness; A step of obtaining heated cold-rolled steel by heating cold-rolled steel from ambient temperature to a first holding temperature in the range of 800 to 900°C at a heating rate of 20°C/second or less; A process of holding heated cold-rolled steel in a furnace at a temperature range of 800 to 900°C for 30 to 300 seconds; A method (100) comprising the steps of: cooling to room temperature at a cooling rate in the range of 20°C/sec to 60°C/sec to obtain a high-strength cold-rolled annealed steel sheet or high-strength cold-rolled annealed steel strip, wherein the high-strength cold-rolled annealed steel sheet contains a microstructure of 10-30% bainite and 70-90% martensite, and the high-strength cold-rolled annealed steel exhibits an ultimate tensile strength in the range of 1400-1800 MPa.
  7. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm, according to claim 6, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.28-0.32, Mn: 1.4-1.5, Si: 0.0-0.2, Al: 0.3-0.5, Mo: 0.25-0.27, with the remainder being iron (Fe) and unavoidable impurities, and the obtained high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1000-1260 MPa, an ultimate tensile strength (UTS) of 1630-1750 MPa, a minimum uniform elongation of 4-5.2%, a yield ratio (YS/UTS) of 0.72-0.75, and a minimum total elongation of 7-9%.
  8. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm, according to claim 7, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.28, Mn: 1.4, Si: 0.2, Al: 0.3, Mo: 0.27, with the remainder being iron (Fe) and unavoidable impurities, and the resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1188 MPa, an ultimate tensile strength (UTS) of 1640 MPa, a minimum uniform elongation of 5.2%, and a minimum total elongation of 8.6%.
  9. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm, according to claim 7, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.32, Mn: 1.5, Si: 0.2, Al: 0.3, Mo: 0.25, with the remainder being iron (Fe) and unavoidable impurities, and the resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1204 MPa, an ultimate tensile strength (UTS) of 1660 MPa, a minimum uniform elongation of 5.1%, and a minimum total elongation of 8.8%.
  10. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm, according to claim 7, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.28, Mn: 1.5, Si: 0.0, Al: 0.5, Mo: 0.25, with the remainder being iron (Fe) and unavoidable impurities, and the resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1147 MPa, an ultimate tensile strength (UTS) of 1600 MPa, a minimum uniform elongation of 5.1%, and a minimum total elongation of 7.3%.
  11. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm, according to claim 7, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.32, Mn: 1.5, Si: 0.0, Al: 0.5, Mo: 0.25, with the remainder being iron (Fe) and unavoidable impurities, and the resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1210 MPa, an ultimate tensile strength (UTS) of 1660 MPa, a minimum uniform elongation of 5.2%, and a minimum total elongation of 7.7%.
  12. A component manufactured from high-strength cold-rolled and annealed steel according to any one of claims 1 to 11, for use in automotive applications.

Description

This invention relates to high-strength cold-rolled and annealed steel sheets or strips having an excellent combination of tensile strength and ductility at room temperature, and more particularly to a method for producing high-strength cold-rolled and annealed steel suitable for the manufacture of structural and safety components, including battery casings for automobiles. Conventional steels such as IF (Interstitial Free), BH (Bake Hardenable), and HSLA (High-Strength Low-Alloy) are well documented in the literature and analyzed in various aspects, including processing, mechanical behavior, microstructure, and texture (as shown in the image below). Their mechanical properties (e.g., strength and ductility) depend on conventional strengthening mechanisms, including grain refinement and solid solution strengthening. Advanced High-Strength Steel (AHSS) is a multiphase steel that utilizes the advanced concept of "composite" microstructure deformation in conjunction with conventional strengthening mechanisms to provide a better combination of mechanical properties. The AHSS family primarily includes duplex (DP) steel, composite (CP) steel, transformation-induced plasticity (TRIP) steel, martensite (MS or MART) steel, and twinning-induced plasticity (TWIP) steel. Apart from conventional strengthening mechanisms, it utilizes composite effects to improve the balance between strength and formability. The microstructure of DP steel consists of a mixture of ferrite and martensite, while composite steel may have a mixture of ferrite, martensite, bainite, and retained austenite. TRIP steel typically has a certain amount of retained austenite with different matrix phases such as ferrite, bainite, and martensite, which transforms into martensite under the influence of stress (transformation-induced plasticity (TRIP) effect), improving the balance between strength and ductility. TWIP steel has a fully austenitic microstructure that generates twins upon application of external stress, thereby providing a better combination of strength and ductility. Among the various types of steel mentioned above, TRIP-type steels are perhaps the most sought-after and researched. However, to retain a viable amount of retained austenite in the final microstructure and extract the advantages of high strength with a better combination of strength and elongation, a minimum amount of Si (>1 wt%) must be used. Si is well known to cause problems during casting and degrade surface quality. These two factors have limited the applications of such steels. This study proposes an alternative method to achieve a better combination of strength and elongation, primarily by utilizing composite effects. This has led to the development of a new type of steel, identified as MB steel, where MB means that the microstructure is a mixture of martensite and bainite. This disclosure aims to overcome one or more of the limitations described above, or any other limitations related to the prior art. A flowchart of a method for producing high-strength cold-rolled and annealed steel according to one embodiment of the present invention is shown. This shows a graph of stress versus elongation obtained during a tensile test of a high-strength cold-rolled annealed steel having the composition MB-1 according to one embodiment of the present invention. This shows a graph of stress versus elongation obtained during a tensile test of high-strength cold-rolled annealed steel having MB-2 composition, according to one embodiment of the present invention. This shows a graph of stress versus elongation obtained during a tensile test of high-strength cold-rolled annealed steel having MB-3 composition, according to one embodiment of the present invention. This shows a graph of stress versus elongation obtained during a tensile test of high-strength cold-rolled annealed steel having MB-4 composition according to one embodiment of the present invention. An optical image of a high-strength cold-rolled annealed steel having the MB-2 composition according to one embodiment of the present invention is shown. The image shows an SEM image of a high-strength cold-rolled annealed steel having the composition MB-2 according to one embodiment of the present invention. This shows an SEM image of a high-strength cold-rolled annealed steel having the composition MB-4 according to one embodiment of the present invention. Unless otherwise specified, the drawings referenced herein should not be understood to be drawn to scale; such drawings are essentially illustrative. A detailed description of various exemplary embodiments of this disclosure is provided herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in detail to clearly convey this disclosure. However, the amount of detail provided herein is not intended to limit the expected variations of the embodiments; rather, the intention is to cover all modifications, equivalents, and alternat