As you may tell because of the “K” at the end of the CPU’s name, the Core i5-7600K has an unlocked multiplier, which means you can overclocking it just by modifying its clock multiplier.
We were able to configure the CPU to run stable at 4.8 GHz (100 MHz reference clock and x48 multiplier), with the original voltages. It may be possible to reach higher frequencies if you “play” with the available adjusts.
However, if you intend overclocking the Core i5-7600K, it is good to use a high-end cooler: using a simple Intel stock cooler while overclocking, the CPU easily reached 85 °C and, at this temperature, reduced the clock automatically. When we used a high-end cooler, the CPU mantained the desired clock all the time.
It is also good to keep in mind that the overclock capability depends on pure luck, since two CPUs of same model can reach different maximum clocks.