Low temperature hardenable steels with excellent machinability
The present invention relates to the application of at least partially bainitic or interstitial martensitic heat treatments on steels, often tool steels or steels that can be used for tools. The first tranche of the heat treatment implying austenitization is applied so that the steel presents a low enough hardness to allow for advantageous shape modification, often trough machining. Thus a steel product is obtained which can be shaped with ease and whose hardness can be raised to a higher working hardness with a simple heat treatment at low temperature (below austenitization temperature).
1. A steel comprising a bainitic and interstitial martensitic microstructure, comprising retained austenite, and/or carbide formers stronger than iron, which are present in the solid solution comprising a composition wherein all percentages are indicated in weight percent, wherein the composition comprises
%Ceq
= 0.16-1.9
%C
= 0.16-1.9
%N
= 0-1.0
%B
= 0-0.6
%Cr
< 1.8
%Ni
= 0-6
%Si
= 0-1.4
%Mn
= 0-3
%Al
= 0-2.5
%Mo
= 0-10
%W
= 0-10
%Ti
= 0-2
%Ta
= 0-3
%Zr
= 0-3
%Hf
= 0-3
%V
= 0-4
%Nb
= 0-1.5
%Cu
= 0-2
%Co
= 0-6,
and iron and trace elements, wherein
% Ceq=% C+0.86*% N+1.2*% B, characterized in that
% Mo+½·% W>2.0
wherein the microstructure comprises less than 18% ferrite, and
characterized by a low scattering structure characterized by a thermal diffusivity higher than 8 mm 2 /s.
2. The steel according to claim 1 , wherein the microstructure comprises of at least 50% vol. bainite.
3. The steel according to claim 1 , wherein the microstructure comprises at least a 50 vol. % interstitial martensite, and
retained austenite is present in 2.5-50% vol., and
carbide formers stronger than iron are present in 2% weight or more in solid solution.
4. The steel according to claim 1 , wherein the microstructure comprises at least 50 vol. % interstitial martensite and
retained austenite is present in less than 2.5% vol., and
carbide formers stronger than iron are present in 3% weight or more in solid solution.
5. The steel according to claim 1 , wherein the bainite is at least 50% high temperature bainite.
6. The steel according to claim 1 , wherein the steel has a carbon content, wherein at least 8% of the carbon content thereof is present in the form of carbides not belonging to the bainitic and/or interstitial martensitic microstructure.
7. The steel according to claim 1 , wherein the steel comprises carbides, wherein at least 30% vol. of the carbides have 50 at % or more iron of all metallic constituents of the carbide.
8. The steel according to claim 1 , wherein the bainite or interstitial martensite present comprises tempered bainite or tempered interstitial martensite.
9. The steel according to claim 1 , characterized in that the sum of the amounts of those elements having an affinity for carbon higher than iron selected from the group consisting of Cr, W, Mo, V, Ti, Nb, Ta, Zr, and Hf is more than 4% in weight.
10. The steel according to claim 1 , characterized in that the microstructure comprises less than 70% of alloyed carbides that can be attained with the chosen composition.
11. The steel according to claim 1 , characterized in that according to a tempering graph of the steel, the martensite and/or bainite present a tempering degree which is smaller than that corresponding to a secondary hardness peak, and a hardness of the steel is below a hardness level of the secondary hardness peak of the steel in an amount of at least 4 HRc.
12. The steel according to claim 1 , characterized in that the martensite and/or bainite present comprise less than a 80% of a nominal % C of the steel.
13. The steel according to claim 1 , characterized in that the martensite and/or bainite present comprise less than a 80% of a nominal % C of the steel in an untempered state.
14. The steel according to claim 5 , wherein the bainite is at least 50% tough high temperature bainite.
15. A steel comprising a bainitic and interstitial martensitic microstructure, comprising retained austenite, and/or carbide formers stronger than iron, which are present in the solid solution comprising a composition wherein all percentages are being indicated in weight percent, wherein the composition comprises:
%Ceq
= 0.16-1.9
%C
= 0.16-1.9
%N
= 0-1.0
%B
= 0-0.6
%Cr
< 1.8
%Ni
= 0-6
%Si
= 0-1.4
%Mn
= 0-3
%Al
= 0-2.5
%Mo
= 0-10
%W
= 0-10
%Ti
= 0-2
%Ta
= 0-3
%Zr
= 0-3
%Hf
= 0-3
%V
= 0-4
%Nb
= 0-1.5
%Cu
= 0-2
%Co
= 0-6,
wherein at least one of the elements selected from the group consisting of Zr, Hf, Nb, and Ta is
present in an amount greater than 0.1%, and
the rest comprises iron and trace elements, and
wherein % Ceq=% C+0.86*% N+1.2*% B, characterized in that
% Mo+½·% W>2.0
wherein the microstructure comprises less than 18% ferrite, and
characterized by a low scattering structure characterized by a thermal diffusivity higher than 8 mm 2 /s.
16. The steel according to claim 15 , wherein the microstructure comprises at least 50% vol. bainite.
17. The steel according to claim 15 , wherein the microstructure comprises at least 50 vol. % interstitial martensite, and retained austenite is present in less than 2.5-50% vol., and carbide formers stronger than iron are present in 2% weight or more in solid solution.
18. The steel according to claim 15 , wherein the microstructure comprises at least 50 vol. % interstitial martensite and retained austenite is present in less than 2.5% vol., and carbide formers stronger than iron are present in 3% weight or more in solid solution.
19. A steel comprising a bainitic microstructure, comprising retained austenite, and/or carbide formers stronger than iron, which are present in the solid solution comprising a composition wherein all percentages are indicated in weight percent, wherein the composition comprises
%Ceq
= 0.16-1.9
%C
= 0.16-1.9
%N
= 0-1.0
%B
= 0-0.6
%Cr
< 1.8
%Ni
= 0-6
%Si
= 0-1.4
%Mn
= 0-3
%Al
= 0-2.5
%Mo
= 0-10
%W
= 0-10
%Ti
= 0-2
%Ta
= 0-3
%Zr
= 0-3
%Hf
= 0-3
%V
= 0-4
%Nb
= 0-1.5
%Cu
= 0-2
%Co
= 0-6,
and iron and trace elements, wherein
% Ceq=% C+0.86*% N+1.2*% B, characterized in that
% Mo+½·% W>2.0
wherein the microstructure comprises at least 50% bainite, and wherein the steel is characterized by a low scattering structure characterized by a thermal diffusivity higher than 8 mm 2 /s.
20. The steel according to claim 19 wherein the microstructure comprises less than 18% ferrite.