High strength spring steel and steel wire
View Patent ↗The present invention provides spring steel used for spring steel wire achieving both high strength and cold coilability and spring steel wire, that is, spring steel containing, by mass %, C: 0.45 to 0.70%, Si: 1.0 to 3.0%, Mn: 0.05 to 2.0%, P: 0.015% or less, S: 0.015% or less, N: 0.0015 to 0.0200%, and t-O: 0.0002 to 0.01 and further limiting Al≤0.01% and Ti≤0.003%. Further, it is characterized by satisfying the following regarding the cementite-based spherical carbides present at an observed plane, an occupied area ratio of grains with a circle equivalent diameter of 0.2 μm or more of 7% or less, a density of presence of grains with a circle equivalent diameter of 0.2 to 3 μm of 1/μm 2 or less, and a density of presence of grains with a circle equivalent diameter 3 μm or more of 0.001/μm 2 or less, having an prior austenite grain size number of #10 or higher and a residual austenite of 15 mass % or less, and having an area ratio of poor regions with a small density of presence of cementite-based carbides of a circle equivalent diameter of 2 μm or more of 3% or less.
1. Heat treated steel wire used for a spring steel containing, by mass %,
C: 0.45 to 0.70%,
Si: 1.0 to 3.0%,
Mn: 0.1 to 2.0%,
P: 0.015% or less,
S: 0.015% or less,
Zr: 0.0001 to 0.0005%
N: 0.0005 to 0.007%,
Cr: 1.16 to 2.5%, and
t-O: 0.0002 to 0.01%,
having a balance of Fe and unavoidable impurities, and further satisfies Al≤0.01% and Ti≤0.003%,
which is rolled, drawn, and heat treated, wherein said steel wire is characterized in that, with respect to cementite-based spherical carbides and alloy-based carbides on an observed plane,
the percentage of an area occupied by carbides having a circle equivalent diameter of 0.2 μm or more is 7% or less,
the density of carbides having a circle equivalent diameter of 0.2 to 3 μm is 1 /μm 2 or less,
the density of carbides having a circle equivalent diameter of 3 μm or more is 0.001/μm 2 or less,
the grain size number of a prior austenite is #10 or higher and the amount of a residual austenite is 15 mass % or less, and
the area ratio of a cementite-based carbide density poor region is 3% or less, and wherein the cementite-based carbide density poor region is a region that has a circle equivalent diameter of 2 μm or more wherein the occupied area ratio of cementite-based carbides in a corroded and recessed region of a microstructure is 60% or less.
2. Heat treated steel wire used for a spring steel as set forth in claim 1 , said heat treated steel wire characterized by further containing, by mass %, one or more of W: 0.05 to 1.0%, Mo: 0.05 to 1.0%, V: 0.05 to 1.0%, Nb: 0.01 to 0.05%, Ni: 0.05 to 3.0%, Co: 0. 05 to 3.0%, B: 0.0005 to 0.006%, Cu: 0.05 to 0.5%, Mg: 0.0002 to 0.01%, Ca: 0.0002 to 0.01%, Hf: 0.0002 to 0.01%, Te: 0.0002 to 0.01%, and Sb: 0.0002 to 0.01%.
3. Heat treated steel wire used for a spring steel containing, by mass %,
C: 0.45 to 0.70%,
Si: 1.0 to 3.0%,
P: 0.015% or less,
S: 0.015% or less,
Zr: 0.0001 to 0.0005%
N: 0.0005 to 0.007%,
Cr: 1.16 to 2.5%, and
t-O: 0.0002 to 0.01%,
having a balance of Fe and unavoidable impurities, and further satisfies Al≤0.01% and Ti≤0.003%,
which is rolled, drawn, and heat treated, wherein said steel wire is characterized in that, with respect to cementite-based spherical carbides and alloy-based carbides on an observed plane,
the percentage of an area occupied by carbides having a circle equivalent diameter of 0.2 μm or more is 7% or less,
the density of carbides having a circle equivalent diameter of 0.2 to 3 μm is 1/μm 2 or less,
the density of carbides having a circle equivalent diameter of 3 μm or more is 0.001/μm 2 or less,
the grain size number of a prior austenite is #10 or higher and the amount of a residual austenite is 15 mass % or less, and
the area ratio of a cementite-based carbide density poor region is 3% or less, and wherein the cementite-based carbide density poor region is a region that has a circle equivalent diameter of 2 μm or more wherein the occupied area ratio of cementite-based carbides in a corroded and recessed region of a microstructure is 60% or less,
wherein the steel of said heat treated steel wire has a uniform tempered martensite microstructure.
4. Heat treated steel wire used for a spring steel as set forth in claim 1 , said heat treated steel wire characterized by containing, by mass %, Zr: 0.0003% or less.
5. Heat treated steel wire used for a spring steel as set forth in claim 1 , said heat treated steel wire characterized by containing, by mass %, Si: 1.6 to 3.0%.
6. Heat treated steel wire used for a spring steel as set forth in claim 1 , said heat treated steel wire characterized by containing, by mass %, Cr: 1.7 to 2.5%.
7. Heat treated steel wire used for a spring steel as set forth in claim 3 , said heat treated steel wire characterized by containing, by mass %, Si: 1.6 to 3.0%.
8. Heat treated steel wire used for a spring steel as set forth in claim 3 , said heat treated steel wire characterized by containing, by mass %, Cr: 1.7 to 2.5%.