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A honeycomb structure constituted by porous cell walls forming a cell group consisting of a plurality of cells, and a honeycomb outer wall surrounding and holding the outermost peripheral cells located at the circumference of the cells, a porosity is 5% to less than 30%, a basic wall thickness (Tc)

A honeycomb structure constituted by porous cell walls forming a cell group consisting of a plurality of cells, and a honeycomb outer wall surrounding and holding the outermost peripheral cells located at the circumference of the cells, a porosity is 5% to less than 30%, a basic wall thickness (Tc) of the cell walls is 0.030 mm≦T c <0.076 mm, an average surface roughness (Ra s ) of the outer wall is 0.5 μm≦Ra s ≦10 μm, and/or an average height (RzDIN s ) on the surface of the outer wall from a local maximum point to a next local minimum point is 5 μm≦RzDIN s ≦50 μm; and a process for producing such a honeycomb structure using a raw material having a specific average diameter. The honeycomb structure satisfies requirements of improved purification ability, erosion resistance and improved canning property at a good balance.

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1. A honeycomb structure constituted byporous cell walls which form a cell group consisting of a plurality of cells adjacent to each other, anda honeycomb outer wall which surrounds and holds the outermost peripheral cells located at a circumference of the cell group, characterized in that a porosit

1. A honeycomb structure constituted byporous cell walls which form a cell group consisting of a plurality of cells adjacent to each other, anda honeycomb outer wall which surrounds and holds the outermost peripheral cells located at a circumference of the cell group, characterized in that a porosity is 5% to less than 30%, a basic wall thickness (T c ) of the cell walls is 0.030 mm≦T c <0.076 mm, an average surface roughness (Ra s ) of the outer wall is 0.5 μm≦Ra s ≦10 μm, and/or an average height (RzDIN s ) on the surface of the outer wall from a local maximum point to a next local minimum point is 5 μm≦RzDIN s ≦50 μm. 2. A honeycomb structure according to claim 1, characterized in that the average surface roughness (Ra s ) is 0.5 μm≦Ra s ≦10 μm and the average height (RzDIN s ) from a local maximum point to a next local minimum point is 5 μm≦RzDIN s ≦50 μm. 3. A honeycomb structure according to claim 1, characterized in that the average surface roughness (Ra s ) is 1.0 μm≦Ra s ≦10 μm and the average height (RzDIN s ) from a local maximum point to a next local minimum point is 10 μm≦RzDIN s ≦50 μm. 4. A honeycomb structure according to claim 1, characterized in that an open frontal area (P) of the honeycomb structure is 83% or more. 5. A honeycomb structure according to claim 1, characterized in that an outer wall thickness (Ts) of the honeycomb structure is 0.076 mm or more. 6. A honeycomb structure according to claim 1, characterized in that an average surface pore diameter of the outer wall is larger than an average inside pore diameter of the outer wall. 7. A honeycomb structure according to claim 1, characterized in that an average surface roughness (Ra c ) of the cell walls is 0.5 μm≦Ra c 10 μm and/or an average height (RzDIN c ) of the cell wall surfaces from a local maximum point to a next local minimum point is 5 μm≦RzDIN c ≦50 μm. 8. A honeycomb structure according to claim 1, characterized in that pores having diameters of 1 μm or more is 90% by volume or more of total pore volume. 9. A honeycomb structure according to claim 1, characterized in that there is a relation of Tr 1 ˜Tr 5-20 )/ T c ≦3.00between the basic cell wall thickness (T c ) and each cell wall thickness (Tr 1 ˜Tr 5-20 ) of cells existing between an outermost peripheral cell taken as a first cell and also as a first starting cell and a first end cell which is any cell of a fifth cell to a twentieth cell all extending inwardly from the first starting cell. 10. A honeycomb structure according to claim 9, characterized in that there is a relation of Tr 1 ˜Tr 5-15 )/ T c ≦3.00between the basic cell wall thickness (T c ) and each cell wall thickness (Tr 1 ˜Tr 5-15 ) of cells existing between an outermost peripheral cell taken as a first cell and also as a first starting cell and a first end cell which is any cell of a fifth cell to a fifteenth cell all extending inwardly from the first starting cell. 11. A honeycomb structure according to claim 9, characterized in that, taking a first cell inward from and adjacent to the first end cell as a second starting cell, cells existing between the second starting cell and a second end cell, which is any cell of a third cell to a fifth cell all extending inwardly from the second starting cell, have a cell wall thickness that is substantially uniform in each cell and gets smaller one by one inwardly and that the smallest cell wall thickness is identical to the basic cell wall thickness (T c ). 12. A honeycomb structure according to claim 9, characterized in that, taking a first cell inward from and adjacent to the first end cell as a second starting cell, cells existing between the second starting cell and a second e nd cell, which is any cell of a third cell to a fifth cell all extending inwardly from the second starting cell, have such a cell wall thickness that a sectional shape of each cell wall is inverse trapezoid and each cell wall thickness gets smaller inwardly and that the smallest cell wall thickness is identical to the basic cell wall thickness (T c ). 13. A honeycomb structure according to claim 9, characterized in that, taking a first cell inward from and adjacent to the first end cell as a second starting cell, cells existing between the second starting cell and a second end cell, which is any cell of a third cell to a fifth cell all extending inwardly from the second starting cell, have such a cell wall thickness that a sectional shape of each cell wall is spool and each cell wall thickness gets smaller inwardly and that the smallest cell wall thickness is identical to the basic cell wall thickness (T c ). 14. A honeycomb structure according to claim 9, characterized in that there is a relation of ≦Tr 1 /T c ≦3.00between the cell wall thickness (Tr 1 ) of outermost peripheral cell and the basic cell wall thickness (T c ), taking outermost peripheral cell as a third starting cell, there is a relation of Tr 1 ˜Tr 5-20 )/ T c ≦3.00between the basic cell wall thickness (T c ) and each cell wall thickness (Tr 1 ˜Tr 5-20 ) of cells existing between the third starting cell and a third end cell which is any cell of a fifth cell to a twentieth cell all extending inwardly from the third starting cell,the cells have a cell wall sectional shape of inverse trapezoid or spool or have a uniform cell wall thickness and their cell wall thicknesses get smaller as the cell is more inward, and the smallest cell wall thickness is identical to the basic cell wall thickness (T c ). 15. A honeycomb structure according to claim 9, characterized in that there is a relation of Tr 1 ˜Tr 5-20 )≦2.50between each cell wall thickness (Tr 1 ˜Tr 5-20 ) and the basic cell wall thickness (T c ). 16. A honeycomb structure according to claim 9, characterized in that there is a relation of Tr 1 ˜Tr 5-20 )≦1.60between each cell wall thickness (Tr 1 ˜Tr 5-20 ) and the basic cell wall thickness (T c ). 17. A honeycomb structure according to claim 9, characterized in that an area of a section of the honeycomb structure perpendicular to the axial direction is 160 cm 2 or more and there is a relation of 1 ˜Tr 10-40 )/T c 3.00≦between the basic cell wall thickness (T c ) and each cell wall thickness (Tr 1 ˜Tr 10-40 ) of cells existing between an outermost peripheral cell taken as a first cell and also as a first starting cell and a first end cell which is any cell of a tenth cell to a fortieth cell all extending inwardly from the first starting cell. 18. A honeycomb structure according to claim 1, characterized in that a value of porosity (%) of part or all of the cell wall portions within 30 mm from one or both end faces of the honeycomb structure is smaller by at least 5 than a value of porosity (%) of other cell wall portion. 19. A honeycomb structure according to claim 18, characterized in that a length of the cell wall portions having a value of porosity lower by at least 5, from the honeycomb end face is not uniform. 20. A honeycomb structure according to claim 1, characterized in that a thickness of the cell walls at one or both end faces of the honeycomb structure is larger than a thickness of the cell walls at other portion. 21. A honeycomb structure according to claim 9, characterized in that the T c is T c ≦0.056 mm. 22. A honeycomb structure according to claim 1, characterized in that the B axis flexural strength is 0.8 MPa or more. 23. A honeycomb structure according to claim 1, characterized by being made of at least one kind of material selected from the group consisting of cordierite, alumina, mullite, silicon nitride, aluminum titanate, zirconia and silicon carbide. 24. A honeycomb structure according to claim 1, characterized in that a sectional shape of the honeycomb structure is circular, elliptical, oval, trapezoidal, triangular, tetragonal, hexagonal or asymmetrical between the right and the left. 25. A honeycomb structure according to claim 1, characterized in that a sectional shape of cells is triangular or hexagonal. 26. A honeycomb structure according to claim 1, characterized by being used as a carrier for catalyst for purification of automobile exhaust gas. 27. A honeycomb structure according to claim 1, characterized in that a catalyst component is loaded on the cell walls of the honeycomb structure, which is assembled into a catalytic converter, and held the outer wall at the outer surface. 28. A honeycomb structure according to claim 1, characterized by being formed so that an intersection of cell walls has a radius of curvature of 1.2 mm or less. 29. A honeycomb structure according to claim 1, characterized by being formed so that an intersection of a cell wall and a honeycomb outer wall has a radius of curvature of 1.2 mm or less. 30. A process for producing a honeycomb structure constituted by porous cell walls which form a cell group consisting of a plurality of cells adjacent to each other, and a honeycomb outer wall which surrounds and holds the outermost peripheral cells located at a circumference of the cell group, in which honeycomb structure a porosity is 5% to less than 30%, a basic wall thickness (Tc) of the cell walls is 0.030 mm≦Tc<0.076 mm, an average surface roughness (Ra s ) of the outer wall is 0.5 μm≦Ra s ≦10 μm, and/or an average height (RzDIN s ) on the surface of the outer wall from a local maximum point to a next local minimum point is 5 μm≦RzDIN s ≦50 μm, the process being characterized in that production is conducted using a raw material containing at least a talc component and a kaolin component, the talc component containing coarse particle talc having an average particle diameter of 7 μm or more and fine particle talc having an average particle diameter of ⅔ or less of that of the coarse particle talc, the kaolin component containing coarse particle kaolin having an average particle diameter of 7 μm or more and fine particle kaolin having an average particle diameter of ⅔ or less of that of the coarse particle kaolin.