If linha.Quant be the type int remove the ToString() is not necessary:
System.Console.WriteLine(string.Format("{0:000000}", 1.ToString())); //1 - não formata
System.Console.WriteLine(string.Format("{0:000000}", 1)); //000001
System.Console.WriteLine(string.Format("{0:000000}", 10)); //000010
System.Console.WriteLine(string.Format("{0:000000}", 67)); //000067
Online Example
But a good one would be with Padleft where:
totalWidth is the integer number that gives the number of characters, and the second parameter if given paddingChar fills in the rest of the string of an informed nature (if not informed, placed space).
System.Console.WriteLine(5.ToString().PadLeft(6, '0')); //000005
System.Console.WriteLine(67.ToString().PadLeft(6, '0')); //000067
System.Console.WriteLine(19.ToString().PadLeft(6, '0')); //000019
System.Console.WriteLine(9.ToString().PadLeft(6, '0')); //000009
Online Example
References: