Carbide-free bainite and retained austenite steels, producing method and applications of same

Patent metadata

Field	Value
Publication number	US-2021047705-A1
Application number	US-201916969318-A
Country	US
Kind code	A1
Filing date	Mar 8, 2019
Priority date	Mar 8, 2018
Publication date	Feb 18, 2021
Grant date	—

Abstract

One aspect, this invention relates to a carbide-free bainite and retained austenite steel including a composition designed and processed such that the carbide-free bainite and retained austenite steel meets property objectives comprising a yield strength in a range of about 1000-2000 MPa, a uniform ductility, a desired total elongation and hole-expansion ratio, a desired level of weldability and an austenite stability designed to have an austenite start temperature Msσ to be equal to an application temperature in range from about 50° C. to −50° C. The property objectives are design specifications of the carbide-free bainite and retained austenite steel.

First claim

What is claimed is:

1 . A carbide-free bainite and retained austenite steel, comprising:
a composition designed and processed such that the carbide-free bainite and retained austenite steel meets property objectives comprising a yield strength in a range of about 1000-2000 MPa, a uniform ductility, a desired total elongation and hole-expansion ratio, a desired level of weldability and an austenite stability designed to have an austenite start temperature M s σ to be equal to an application temperature in range from about 50° C. to −50° C., wherein the property objectives are design specifications of the carbide-free bainite and retained austenite steel.

2 . The carbide-free bainite and retained austenite steel of claim 1 , wherein the composition is processed with a cooling and partitioning treatment.

3 . The carbide-free bainite and retained austenite steel of claim 1 , wherein the composition comprises carbon (C) no more than 0.4 wt %, silicon (Si) no less than 1.0 wt %, and iron (Fe) in balance.

4 . The carbide-free bainite and retained austenite steel of claim 2 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and molybdenum (Mo) in a range of about 0.4-0.8 wt. %.

5 . The carbide-free bainite and retained austenite steel of claim 2 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and chromium (Cr) in a range of about 0.1-0.9 wt. %.

6 . The carbide-free bainite and retained austenite steel of claim 1 , wherein the property objectives further comprises a carbon concentration in austenite, C γ , in a range of about 1.0-1.8 wt. %.

7 . The method of claim 1 , wherein the application temperature is about 5° C., or about −20° C.

8 . A method for producing a carbide-free bainite and retained austenite steel, comprising:
providing an iron (Fe) alloy containing a composition designed according to property objectives of the carbide-free bainite and retained austenite steel, wherein the property objectives are design specifications of the carbide-free bainite and retained austenite steel; and heat-treating the alloy to a temperature above A c3 , wherein A c3 is a temperature at which a transformation from ferrite into austenite is finished; succeeding quenching the heat-treated alloy to a bainite region at a temperature between M s and B s , wherein M s is a temperature at which a martensitic transformation starts in the alloy, and B s is a temperature at which a coupled diffusional/displacive bainitic transformation starts the alloy; and optimally cooling the quenched alloy to form to form the carbide-free bainite and retained austenite steel that meets the property objectives, wherein a cooling ratio of the optimally cooling step is precisely controlled so that the temperature of the alloy continues to be slightly above the M s temperature which keeps decreasing during the optimally cooling step.

9 . The method of claim 8 , wherein the heat-treating step is performed with an austenization or hot rolling treatment.

10 . The method of claim 8 , wherein the heat-treating step is performed with a hot rolling treatment and subsequently a cold rolling treatment and a solution treatment.

11 . The method of claim 8 , wherein the optimally cooling step is performed with gradually cooling or step-wise cooling.

12 . The method of claim 8 , wherein the property objectives comprises a yield strength in a range of about 1000-2000 MPa, a uniform ductility, a desired total elongation and hole-expansion ratio, a desired level of weldability and an austenite stability designed to have an austenite start temperature M s σ to be equal to an application temperature in range from about 50° C. to −50° C.

13 . The method of claim 8 , wherein the composition comprises carbon (C) no more than 0.4 wt %, silicon (Si) no less than 1.0 wt %, and iron (Fe) in balance.

14 . The method of claim 13 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and molybdenum (Mo) in a range of about 0.4-0.8 wt. %.

15 . The method of claim 13 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and chromium (Cr) in a range of about 0.1-0.9 wt. %.

16 . A method for designing a carbide-free bainite and retained austenite steel, comprising:
defining property objectives of the carbide-free bainite and retained austenite steel, wherein the property objectives are design specifications of the carbide-free bainite and retained austenite steel; designing a composition of the carbide-free bainite and retained austenite steel according to the property objectives; and processing the composition to form the carbide-free bainite and retained austenite steel that meets the property objectives, wherein the processing step is performed with a cooling and partitioning process.

17 . The method of claim 16 , wherein the processing step comprises solidifying the composition to form an alloy; and reheating the alloy.

18 . The method of claim 17 , wherein the cooling and partitioning process comprises:
heat-treating the alloy to a temperature above A c3 , wherein A c3 is a temperature at which a transformation from ferrite into austenite is finished; succeeding quenching the heat-treated alloy to a bainite region at a temperature between M s and B s , wherein M s is a temperature at which a martensitic transformation starts in the alloy, and B s is a temperature at which a coupled diffusional/displacive bainitic transformation starts the alloy; and optimally cooling the quenched alloy to form to form the carbide-free bainite and retained austenite steel that meets the property objectives, wherein a cooling ratio of the optimally cooling step is precisely controlled so that the temperature of the alloy continues to be slightly above the M s temperature which keeps decreasing during the optimally cooling step.

19 . The method of claim 18 , wherein the heat-treating step is performed with an austenization or hot rolling treatment.

20 . The method of claim 18 , wherein the heat-treating step is performed with a hot rolling treatment and subsequently a cold rolling treatment and a solution treatment.

21 . The method of claim 18 , wherein the optimally cooling step is performed with gradually cooling or step-wise cooling.

22 . The method of claim 16 , wherein the property objectives comprises a yield strength in a range of about 1000-2000 MPa, a uniform ductility, a desired total elongation and hole-expansion ratio, a desired level of weldability and an austenite stability designed to have an austenite start temperature M s σ to be equal to an application temperature in range from about 50° C. to −50° C.

23 . The method of claim 16 , wherein the composition comprises carbon (C) no more than 0.4 wt %, silicon (Si) no less than 1.0 wt %, and iron (Fe) in balance.

24 . The method of claim 23 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and molybdenum (Mo) in a range of about 0.4-0.8 wt. %.

25 . The method of claim 23 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and chromium (Cr) in a range of about 0.1-0.9 wt. %.

26 . A method for designing a carbide-free bainite and retained austenite steel, comprising:
determining a composition, and produci

Assignees

• Univ Northwestern

Inventors

• Nishioka Kazuhiko
• Olson Gregory B

Classifications

• C21D8/02

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

• C22C38/22 Primary

  with molybdenum or tungsten · CPC title

• C21D2211/001

  Austenite · CPC title

• C21D8/0226

  Hot rolling · CPC title

• C22C38/18

  containing chromium · CPC title