According to the Rogue Trader core rulebook about ship weapons:

"Strength: This is the maximum number of hits a macrobattery or lance can land on an enemy ship." (Rogue Trader Errata v1.4)

"Crit Rating: This is the number of successes the shot must have to score a critical hit on the target." (Rogue Trader, pg. 219)

Looking at some of the ship weapons, many have a strength score lower than their crit rating. Is it possible for these type of weapons to score a critical hit on their own, or must they be combined in a salvo for them to land a critical hit?

Also, can multiple lances be formed to make a salvo as well?

Say you have a macrocannon that's Strength 2, Crit Rating 3. 2 degrees of success on your BS test gets you the two hits. Three degrees gets you the critical hit as well. Additional degrees of success above the Strength aren't discarded, there's just a maximum number of hits each weapon can achieve.

Lances can't salvo, it's a unique feature of macrocannons.

Also remember that if you salvo you can only score one critical hit, regardless of how many macrocannons you salvo with.

Say you have a macrocannon that's Strength 2, Crit Rating 3. 2 degrees of success on your BS test gets you the two hits. Three degrees gets you the critical hit as well. Additional degrees of success above the Strength aren't discarded, there's just a maximum number of hits each weapon can achieve.

Lances can't salvo, it's a unique feature of macrocannons.

So a lance weapon that has a strength of 1 and a crit rating of 3 will crit on three degrees of success, but only hit once. Right?

That is correct, yes.