Were Chinese Helmets a One Peice Cast , or were they forged into shape like roman helmets.

Depends on the era

A lot of the helmets were composites of multiple iron or bronze scales...

Did they ever just mass produce a One Peice helmet from cast iron ,like the WW2 helmets.

On this topic of helmets, does anyone have images of Qin army helmets or even warring state period helmets?

Would these helmets be made out of bronze, or iron?

They used Bronze poppet.

From warring states period they used Copper->Bronze.

There is an image of the Qin style of helmet on this thread;
http://www.chinahistoryforum.com/index.php?showtopic=11406

The helmet is made of brittle stone sheets but the real versions are assumed to be leather, as is the buried armies lamellar too by most expert commentary. This is because iron armour has not been found in association with Qin sites while other Warring States were praised for the production of iron weapons in particular.

The question this thread is based on is so unspecific I didn't bother to respond. A study of each period would be better so 'Yes' & 'No' answers are not forcoming.
There is a thread nearby on helmets in East Zhou and many of these questions for at least the earliest period could be answered by a search of such threads.

Most armour in ancient China was leather. Bronze single piece helmets which exist are uncommon and bronze is most often found simply as armour ornamentation, small and decorative (domes and various shapes) and not effective as armour. Some examples of these I have shown on CHF as small domes are fairly common. Some bronze plates that possibly attach to armour do exist but these are debatable since they are not mentioned by academics.
In this way most armour is organic leather made of numerous types of animals. Most have been lost to decay but some rare suits have survived for study. There are few examples but these are shown in most texts on ancient warfare and in reconstuctions.

Lamellar leather helmets existed by the late Spring and Autumn (if not earlier). Bronze cast helmets existed in late Shang and West Zhou and continue into the Warring States period but great numbers have not turned up. Organic armour would be cheaper, lighter and also practical when laquered into a hard plate. I have never heard of bronze lamellar (helmets made of plates) and would say with some certainty that they didnt exist. Bronze is even heavier than iron (slightly) but some bronze can be very heavy depending on %. Lead is routinely added to Chinese bronzes of the period with some being more the 10% of the alloy. This reason may have limited the use of bronze as armour compared to leather and also the fact it is expensive as a material.

Iron helmets of lamellar plates exist in the late Warring States period and Han era metal helmets are lamellar constructions too.
One piece cast iron helmets (or forged one piece) do not exist at this time, which is odd since casting of iron existed from the mid-East Zhou. Only early steels, folded, seem to have been used for iron armour and these are in the form of linked plates.
The lamellar helmets of late East Zhou were folded 'steel' of a lesser quality to Han steel however. These are also rather uncommon and associated with some states and not others. Leather armour was still being used at the beginning of the Han but was largely replaced by steel/iron by mid-West Han. Leather always remained though and even in the Tang there was use of leather armour. The style of Han helmets varies a lot but most true helmets are lamelar. The headgear shown on ceramic warriors is hard to tell if they are just caps for hair or simple one piece leather helmets (shown on the recent thread on helmets). They are not mentioned in Albert Diens armour study so may just be a fashion item rather than protection.
Of the later dynasties other people can offer comment but many of these points have already been discussed on CHF so untill the question is made more specific there is little to add.