------
## 最终结论
```python
from collections import deque

def min_steps_to_collect_all_keys(grid):
    # Directions for moving in 4 cardinal directions
    directions = [(0, 1), (1, 0), (0, -1), (-1, 0)]
    
    # Set up grid dimensions, starting position, and total keys.
    m, n = len(grid), len(grid[0])
    all_keys = 0
    start_x = start_y = -1

    # Determine the starting point and the number of keys
    for i in range(m):
        for j in range(n):
            if grid[i][j] == '@':
                start_x, start_y = i, j
            elif 'a' <= grid[i][j] <= 'f':
                all_keys |= (1 << (ord(grid[i][j]) - ord('a')))
    
    # BFS queue: (x, y, keys)
    queue = deque([(start_x, start_y, 0)])
    visited = set((start_x, start_y, 0))
    
    steps = 0
    
    # While there are states in the queue, explore each neighbor.
    while queue:
        for _ in range(len(queue)):
            x, y, keys = queue.popleft()
            
            # If all keys are collected
            if keys == all_keys:
                return steps
            
            # Explore neighbors
            for dx, dy in directions:
                nx, ny = x + dx, y + dy
                
                if 0 <= nx < m and 0 <= ny < n:
                    cell = grid[nx][ny]
                    
                    # Check if it's a wall
                    if cell == '#':
                        continue
                    
                    # Check if it's a lock and we don't have the key
                    if 'A' <= cell <= 'F' and not (keys & (1 << (ord(cell) - ord('A')))):
                        continue
                    
                    # Collect the key if it's a key
                    new_keys = keys
                    if 'a' <= cell <= 'f':
                        new_keys |= (1 << (ord(cell) - ord('a')))
                    
                    # If this state hasn't been visited yet
                    if (nx, ny, new_keys) not in visited:
                        visited.add((nx, ny, new_keys))
                        queue.append((nx, ny, new_keys))
        
        steps += 1
    
    # If we exit the while loop without having collected all keys, return -1
    return -1

# Example usage:
print(min_steps_to_collect_all_keys(["@.a.#","###.#","b.A.B"])) # Output: 8
print(min_steps_to_collect_all_keys(["@..aA","..B#.","....b"])) # Output: 6
```