Susan Park in Meishan City plans to lay square bricks in the fitness area. At present, two engineering teams, A and B, are bidding, and the cost of bricklaying for Team A is Y.

(1) As can be seen from the figure, the function image passes through points (500,2800), (1000,4800),

When 0≤x≤500, let the functional relationship be y=ax,

Then 500a=2800,

The solution is a=5.6,

Therefore, y=5.6x(0≤x≤500),

When x > 500, let the functional relationship be y=bx+c,

500b+c=2800 1000b+c=4800，

The solution is b=4 c=800.

Therefore, y = 4x+800 (x > 500),

To sum up, the functional relationship between y and x is y = 5.6x (0 ≤ x ≤ 500) 4x+800 (x > 500);

(2)y A =4× 1600+800=7200 (yuan),

Y b = 1600k，

When y a > y b, 7200 < 1600k,

That is, k > 4.5,

Therefore, when k > 4.5, it is more cost-effective for the park to choose B engineering team for construction.