Shift from iron to steel is easy, as it is easy to discover that adding carbon to iron would create steel, and adjusting the quantities and using different heating methods create different hardness and softness. This needs to be done by trial and experimentation, and we modern peoples should not underestimate that ancient peoples can be smart and resourceful too.
Mixing iron with different quantities of carbon and phosphorus led to creation of the steel, and then the steel sword, in particular the dao, with a construction method that uses a soft core, the use of folding steel, differential heat treatment --- you apply clay on the sword while forging it, so different areas of the sword have different heat levels. Similar methods were also used with bronze, and the techniques were applied to iron and steel. This became the way Chinese swords are made, and same techniques are passed on to Korea and Japan, laying basis for katana.
Steel is actually very hard to make, especially consistently.
The issue until the 1800s is mostly temperature control
How to control temperature without thermometers so that the steel is in the right phase? how to ensure that the carbon from the fuel does not contaminate the crucible and transform the steel into a different steel that you want? Tempering, forging, the clay is to insulate the steel so that when it cools, it is slower and the crystal is larger so softer.. these are all techniques but at the end of the day, ancient sword making is a hit or miss. it is like the modern chipset industry, a few example will come out just right and given to lords, many of them will have minor defect and be arm the elite troops and the ones which turned out just okay would be issued to regular troops.
Even the famed katana which the ming and qing brought alot of, is known to break in combat and that is the reason why samurais will carry a backup short swords and I believe some historical records said that some samurai will bring half a dozen katana to battle (by retainers of course, and passed to him if his katana was broken)
Thinner, harder swords are more brittle and prone to break. Making edges extra sharp can make these edge prone to cracking.
Sword longevity is not a problem unless you are a sword collector, and even then you are not likely to use the sword so many times.
So the steel QC varies, so what? How many times do you expect the sword to be used or last in the battlefield? Since somehow you have to expect the soldier to survive in the first place. The swords themselves are mainly backup to polearms.
To deal with the variance of steel quality, this led to dao being shorter, broader, and thicker with flaring ends as time goes on. The long thin proto-katana two handed ring dao would give way to dadaos and zhanmadaos. Chop! Chop! This led to the far better and more popular image of the dao. During WW2, dadaos are being made ranging from steel taken from railway tracks to pots and pans, yet capable of breaking through Japanese military issued gunto and decapitate opponent. Where is the QC in those dadaos? I heard that Japanese soldiers carrying ancestral katana did much better on melee combat than those using gunto, and this led to the practice of Japanese soldiers going away to war with either ancestral katana or families paying a skilled swordsmaker to make a high quality sword as a going away present and insurance for their son.
Japanese swords or nihon-to are well known for their craftmanship, and they were being seriously collected as early as the Song. But during the Mongol invasion of Japan, the Japanese swords were breaking against Chinese and Mongolian armor, leading to the back of the swords being made thicker.
Longevity to last a battle. if longevity doesn't matter than viking warriors will not pay for the famed Ulfberht swords, the fact is sword breakage is fairly common on the battlefield and when your opponents sword is more likely to break then you are more likely to kill him then he kill you.
And not all swords are made to do the same thing. you have chopping swords like the dao which need the weight and thickness in the blade to carry through. or slicing swords like katana, or piercing swords like the jian or hacking swords like the broad sword. Katanas and jians needs a sharp edge to be effective and to have that will require a brittle edge that needs to be protected with a soft core. thats why katanas have diamond edges and jians have parabolic edges; and the smiths prefer folding and inserting a high carbon edge. Broad swords needs to be hack proof and therefore twist cores are preferred, the cords are thicker and the edges are not sharp so there is better edge retention.
I won't take the ability to decapitate to be meaningful, can a foil sword decapitate? probably not as some doesn't even have an edge, its purpose is to punch a hole through chain mail and gaps in plate. a fan blade propeller from a kitchen exhaust fan can decapitate.
Also, railway steel is not bad steel, it is actually well forged steel. pots and pans, i doubt is true as pots and pans of that era in china are mainly cast iron. so most records says railway steel.
[/quote]Now this being all said, swords are secondary weapons for samurais and similarly for Chinese warriors. primary weapons are bows and spears.
yeah, just a comment: before you even start with steel, you need crude iron (out of a blast furnace) prepared "consistently"Steel is actually very hard to make, especially consistently.
I mean, i did not say that you expect swords to last years, I have been saying that it needs to be able to last a battle so there is no argument there. the concept of good enough is vis-a-vis your opponent. iron sword > bronze sword, steel > iron, forged steel > steel and so on.
And a cross bow is technically a bow isn't it? so my statement is correct, had I said slings like the romans used, then yes you are right. in this sense even Chinese slingshots are recuve bows firing pellets.
Also, railway steel was imported from Russia, England and the sorts, so yes it was good quality steel. That is why you have manchurian railway incident - which was built by japan, the chinese eastern railway built by Russia - which sparked the russo japanese war. the kowloon canton railway built by the UK. so those steel are good steel comparatively.
And be it if swords are the primary armament; lets look at some historical units from antiquity:
Primary Weapon: Doru spear
Secondary Weapon: Xiphos sword
Armor: linen armor, bronze helmet and grieves
Shield: large round shield
Republic roman legionaries
Primary weapon: Gladius sword/dagger
Primary missile weapon: Pilum javelin X2
Secondary misslie weapon: sling with lead bullets
Armor: single chest plate of bronze or iron
Shield: rectangular curved shield
Eastern Zhou heavy infantry
Primary weapon: pole arm Ge/Spear
Secondary weapon: iron sword
Armor: animal hide and helmet
So unless you are suggesting that the ancient Chinese infantryman is less physically able to carry the weapons and gears of their eastern counterparts, then swords as a secondary weapon is not so hard of a concept to grasp.
There are swords only infantry such as specialized double handed sword infantry like zweihander, or skirmishers but they are not numerous and typical. even infantry using zhanmao dao vs. cavelarty is highly reliant on the polearm formation to break up cavalry charges to be effective. It should be evident that a wall of lancers will hit the infantry with the double handed sword first with their lances before the infantry can swing their sword. the double handed sword is only effective when the cavalry run past the infantry squares.
what people generally forget, and i blame TV and movies, is that the Chinese battlefield is not made of cold steel for a very long time. the Song had invented cannons and the the last 1000 years, three barrel pole guns are very common during the song and the ming.
This is from the fire dragon manual from 1300s/1400s
you see Chinese musketry and the secondary sword on their belts.
But we are digressing, my point is, making steel is hard, making good steel is harder.
There is a really huge difference in terms of performance, capability, or usage of different bow types. If you compare the short bow, to the long bow, to the composite bow, then to the crossbow, they really are not the same thing even if they are all projectile weapons. Even crossbows are not all the same. You have manually loaded crossbows, foot loaded crossbows, and ones which use a winching mechanism which all have different projectile speed/range and reload times.
The difference between a bow and a crossbow is also akin to the difference between a bow and a musket in that you do not need well trained troops to operate one. i.e. to use a bow, especially a stronger bow like a longbow or a composite bow, you require many months or years of training to gain the muscular strength and accuracy to fire one. To maintain that muscular strength you need to upkeep those troops constantly not just at time of war. The British did this by instituting the yeoman class for their longbow men. This was a class which owned its own plot of land to cultivate with the provision that they had to train the bow weekly and be available at time of war.
If you look at troops in roughly the same time period as the Qin, like the Persian Immortal army, they also had a complex military structure.