Low carbon steel having improved hardness and methods of making the same

Patent metadata

Field	Value
Publication number	US-11761067-B2
Application number	US-201916601024-A
Country	US
Kind code	B2
Filing date	Oct 14, 2019
Priority date	Oct 15, 2018
Publication date	Sep 19, 2023
Grant date	Sep 19, 2023

Abstract

Provided herein are techniques for making low-carbon steels with high surface hardness. A technique includes heating a low-carbon steel precursor material in a furnace to form molten steel material, increasing the free oxygen content of the molten steel material to a predetermined level, and then solidifying the molten steel material having the predetermined oxygen level to produce a steel structure by cooling the molten steel material at a predetermined cooling rate. The predetermined oxygen level and the predetermined cooling rate are effective to produce the low-carbon steel with a high surface hardness. The low-carbon steel may have inclusions smaller than about 1 μm.

First claim

We claim:

1. A method for making a low-carbon steel, the method comprising:
heating a low-carbon steel precursor material in a furnace to form molten steel material;
increasing the free oxygen content of the molten steel material to a predetermined level from 25 ppm to 45 ppm; and then
solidifying the molten steel material having the predetermined oxygen level to produce a low-carbon steel structure by cooling the molten steel material at a predetermined cooling rate,
wherein the low-carbon steel structure has a surface hardness of at least 4.0 GPa Vickers immediately after cooling, and
wherein the predetermined cooling rate is at least 2500 K/s.

2. The method of claim 1 , wherein the low-carbon steel structure has a surface hardness of at least 4.2 GPa Vickers immediately after cooling.

3. The method of claim 1 , wherein the low-carbon steel structure comprises inclusions smaller than about 1 μm.

4. The method of claim 3 , wherein the inclusions have sizes in a range from 0.5 to 0.7 μm.

5. The method of claim 3 , wherein the inclusions are present in the low-carbon steel in an area density up to 600 per mm 2 .

6. The method of claim 3 , wherein the inclusions comprise multiple-component, Ti-containing oxides.

7. The method of claim 1 , wherein increasing the free oxygen content comprises adding FeO to the molten steel material.

8. The method of claim 7 , wherein the FeO is added in an amount effective to increase the free oxygen content to 38 ppm.

9. The method of claim 7 further comprising adding to the molten steel material one or more of a Ferromanganese (FeMn) alloy, a Si Ferrosilicon (FeSi) alloy, or a Ti Ferrotitanium (FeTi) alloy.

10. A method for making a steel structure, the method comprising:
heating a low-carbon steel precursor material in a furnace to form molten steel material;
adding FeO to the molten steel material in an amount effective to produce a predetermined level of free oxygen content of between 25 ppm and 45 ppm; and
adding one or more of FeTi, FeMn, or FeSi to the molten steel material in an amount effective to produce nucleation sites for acicular ferrite in an area density up to 600 per mm 2 ; and then
solidifying the molten steel material to produce the steel structure by cooling the molten steel material at a predetermined cooling rate effective to produce inclusions smaller than about 1 μm,
wherein the predetermined cooling rate is greater than or equal to 2500 K/s, and
wherein the steel structure has a surface hardness of at least 4.0 GPa Vickers immediately after cooling.

11. The method of claim 10 , wherein the steel structure is a sheet.

12. The method of claim 10 , wherein the steel structure comprises an ultrahard surface layer.

13. A method for making a low-carbon steel, the method comprising:
heating a low-carbon steel precursor material in a furnace to form molten steel material;
adding to the molten steel material one or more of a Ferromanganese (FeMn) alloy, a Si Ferrosilicon (FeSi) alloy, or a Ti Ferrotitanium (FeTi) alloy;
increasing the free oxygen content of the molten steel material to a predetermined level from 25 ppm to 45 ppm; and then
solidifying the molten steel material having the predetermined oxygen level to produce a steel structure by cooling the molten steel material at a predetermined cooling rate,
wherein the steel structure has a surface hardness of at least 4.0 GPa Vickers immediately after cooling,
wherein the predetermined cooling rate is greater than or equal to 2500 K/s,
wherein the steel structure comprises inclusions smaller than 1 μm, and
wherein the inclusions are present in the steel structure in an area density up to 600 per mm 2 .

Assignees

• Univ Florida State Res Found Inc

Inventors

• Li Huigai
• Han Ke
• Xin Yan
• Zheng Shaobo
• Wang Liuxing
• Zhai Qijie

Classifications

• C22C38/004 Primary

  Very low carbon steels, i.e. having a carbon content of less than 0,01% · CPC title

• C22C33/06 Primary

  using master alloys · CPC title

• C21D9/46

  for sheet metals · CPC title

• C22C38/02

  containing silicon · CPC title

• C22C38/04

  containing manganese · CPC title