Sliding member

1. A sliding member comprising:

a steel back metal layer; and

a sliding layer on the steel back metal layer, the sliding layer including a porous sintered layer and a resin composition,

wherein the porous sintered layer includes a Ni—P alloy part and Fe or Fe alloy granules,

wherein the Ni—P alloy part functions as a binder for binding the Fe or Fe alloy granules with one another and/or for binding the Fe or Fe alloy granules with the steel back metal layer, and

wherein the steel back metal layer is made of a carbon steel including 0.05 to 0.3 mass % of carbon, and comprises:

a non-austenite-containing portion in a central portion of a thickness direction of the steel back metal layer, the non-austenite-containing portion having a structure of a ferrite phase and perlite formed; and

an austenite-containing portion in a surface portion of the steel back metal layer facing the sliding layer, the austenite-containing portion having a structure of a ferrite phase, perlite and an austenite phase.

2. The sliding member according to claim 1 , wherein Ni atoms of the Ni—P alloy part is diffused into the austenite-containing portion.

3. The sliding member according to claim 1 , wherein the austenite-containing portion has an average thickness of 1 to 30 μm.

4. The sliding member according to claim 1 , wherein a ratio of the austenite phase in the austenite-containing portion is 0.05 to 3 volume %.

5. The sliding member according to claim 1 , wherein the austenite phase in the austenite-containing portion has an average grain size of 0.5 to 5 μm.

6. The sliding member according to claim 1 , wherein a ratio of the perlite in the austenite-containing portion is at least 25% less than a ratio of the perlite in the non-austenite-containing part.

7. The sliding member according to claim 1 , wherein the Ni—P alloy part has a composition comprising:

9 to 13 mass % of P;

optionally one or more selected from 1 to 4 mass % of B, 1 to 12 mass % of Si, 1 to 12 mass % of Cr, 1 to 3 mass % of Fe, 0.5 to 5 mass % of Sn and 0.5 to 5 mass % of Cu; and

the balance of Ni and inevitable impurities.

8. The sliding member according to claim 1 , wherein a ratio of the Ni—P alloy part in the porous sintered layer is 5 to 40 parts by mass in relation to 100 parts by mass of the porous sintered layer.

9. The sliding member according to claim 1 , wherein the Fe or Fe alloy granules are made of a carbon steel including 0.3 to 1.3 mass % of carbon,

wherein the Fe or Fe alloy granules have

a structure of a ferrite phase, perlite and an austenite phase; or

a structure of a ferrite phase, a mixture of perlite and cementite, and an austenite phase, and

wherein the Fe or Fe alloy granules have a high austenite phase portion in a surface portion of the Fe or Fe alloy granules, the high austenite phase portion has a ratio of an austenite phase at least 20% higher than a ratio of an austenite phase in a central portion of the Fe or Fe alloy granules.

10. The sliding member according to claim 9 , wherein the Fe or Fe alloy granules have an average grain size of 45 to 180 μm.

11. The sliding member according to claim 9 , wherein Ni atoms of the Ni—P alloy part is diffused in a surface of the Fe or Fe alloy granules.

12. The sliding member according to claim 9 , wherein the high austenite phase portion has a thickness of 1 to 30 μm.

13. The sliding member according to claim 9 , wherein a ratio of the austenite phase in the high austenite phase portion is 0.05 to 3 volume %.

14. The sliding member according to claim 9 , wherein the austenite phase in the high austenite phase portion has an average grain size of 0.5 to 5 μm.

15. The sliding member according to claim 9 , wherein a ratio of the ferrite phase in the high austenite phase portion is at least 25% higher than a ratio of the ferrite phase in the central portion of the Fe or Fe alloy granules.