Ooh! This may be, of all the questions I've answered on this sub, the one I am most qualified to answer - forget my flair, this is what my degree is in. Now, this question is really more technological than historical, and very much blows past the twenty year rule in some ways, but let's dive in.

The simple reason you don't see serious mass issue of steel armor - I am not counting limited-issue items like SN-42 - in the 20th century or the modern era is that it's too dang heavy. The US Army's Natick Laboratory established during the Vietnam-era Variable Body Armor program that there is a maximum effective armor weight, beyond which point the soldier's mobility is unacceptably compromised. The Natick study normalized these weights as areal densities for each plate, to account for different soldier sizes and plate thicknesses. The critical areal density for infantry body armor is 7 pounds per square foot. The areal density for rifle-resistant AR500 steel plate? About 11 lbs/ft****^(2). Trying to fight and maneuver with this much weight, on top of the weight of the rest of the soldier's fighting load, is just not feasible.

The SN-42 breastplate is known to have weighed 3.3kg (~7.25 lbs), according to a report from 68th Army in early 1943. I do not know the surface area or areal density of the plate, unfortunately. However, I do know that comparing SN-42 to commercially available AR500 armor is apples and oranges. The commercially available AR500 armor, recently popularized in some circles within the American firearms community, is designed to stop rifle fire. The SN-42 breastplate, on the other hand, is noted to have been resistant to 9mm submachine gun fire, grenade fragments, and (sometimes) rifle fire at distances of 300m or more. In modern times, soft body armor like Kevlar is sufficient to protect against submachine guns and fragments; these armors have areal densities as low as 0.7 lbs/ft^(2).

So if not steel, then what? Why has plate-based, rifle-resistant armor only become a thing relatively recently? Well, since the middle of the 20th century there has been a lot of research into high-hardness ceramic armor materials. There are some neat experimental programs during the early periods of this that I am not super solid on, but in useful terms the result of this research has been the American SAPI/ESAPI/XSAPI family of armor plates, fielded ubiquitously in the last twenty years. Ceramic armors are highly sensitive to processing and manufacturing precision, which is probably the biggest technological barrier to introducing ceramic armors earlier than the mid-late 20th century. There are also technical barriers regarding the engineering community's understanding of exactly how bullets penetrate brittle armor materials; there's not a suuuuper robust understanding of this behavior until Cline and Wilkens' foundational research in the 1960s, but that is way into the weeds.

EDIT: For completeness' sake, I should also discuss some non-American developments. During the late 70s and 80s, the Soviets fielded the 6B2 armor system, which as far as I can determine uses titanium plates to protect against rifle fire. Subsequent Soviet and Russian systems have used titanium, and more recently ceramics. The Soviets seem to have pursued titanium for the weight savings, as well as the fact that Russia has more access to titanium than most Western countries. I don't have a great handle on Soviet/Russian stuff here, because the source material is frustratingly opaque.

Sources and Further Reading:

Crouch, Ian G. The Science of Armour Materials, Woodhead Publishing in Materials 2017.

http://www.tankarchives.ca/2013/10/soviet-infantry-protection.html - Peter Samsonov's (/u/TankArchives) primary-source discovery on the SN-42 breastplate is referenced here.

Also pinging /u/alasdhairM because I know he's doing some primary source work as well on the history of American body armor development.

/u/Jon_Beveryman went into pretty good detail as to the "why not steel" side of things, so I'll try to explain why rifle protective body armor wasn't more widely issued prior to the 1990s/2000s.

First, rifle-protective personal body armor has existed since at least the First World War. I'll go into more detail about that below, but generally when you have people conducting close-quarters battle at high intensities, like the trench raiding during the first world war, or when Delta and the SAS/SBS are doing three raids a night in Iraq, you have rapid and effective developments in personal body armor driven by the needs of the end-user.

There are mentions of limited-production systems proof against rifle fire in Bashford Dean's Helmets and Body Armor in Modern Warfare. The most prominent of these are the Corelli and Roneo-Miris "body shields", and a prototype by a Col. C. F. Close. While the metallurgy isn't discussed, their proof to full metal jacket 8mm Mauser from a Gew. 1898 suggests that they approximate the performance of modern steel rifle armor. The thickness of the plates, the smallest of which is 11"x14" and seventeen pounds, is likely also similar. Interestingly, Col. Close's plate carrier, which is more or less what these are, incorporated an anti-spall coating of some form of fabric. Discussion of these can be found on pages 126 and 127 of his book.

Dean's work further supports the weight arguments posed by the Variable study 47 years later, noting:

In general, it is evident that English inventors made strenuous efforts to solve the armor problem by devising a defense which should be light in weight and easy to wear. Their best experts declared that no armor could be used successfully which was heavier than six pounds.

Bashford Dean, Helmets and Body Armor in Modern Warfare (New Haven, CT: Yale University Press, 1920), p.118)

During the Second World War, most of armor development was focused on the helmet, although an armor system for ground troops was type classified as the M12, which consisted of aluminum alloy thoracic plates and nylon fragmentation protection. There was also development of a doron, or autoclaved fiberglass, based vest that was issued on Okinawa. By Korea, various nylon-based fragmentation armor systems had been developed that would evolve through to the PASGT system.

The primary impedance to the issuance of rifle protective personal body armor was that it was prohibitively expensive to widely issue. The 1967-1970 Variable Body Armor for Ground Troops, for example, was extremely expensive -- it ran over $800 per vest, equivalent to $6,000 today, and even though 42,000 vests were produced, and 28,000 were sent to Vietnam by 1970, they weren't widely issued to infantry units and the remainder have been sitting in a warehouse ever since. Variable Body Armor's plates were extremely interesting, a reaction-bonded boron carbide strikeface that had a multi-curve profile and was designed to resist 7.62x51mm M61 AP at 100yd, and 7.62x39mm mild steel cored ball at 10yd, at only 6.0lb/ft^2. They were used by Natick Labs for various programs, and even considered for issue in the late 1980s and early 1990s as an off-the-shelf armor system, but the idea was shelved, most likely due to the fall of the Soviet Union.

Even SOF units like Delta or the direct action-oriented elements of 10th Special Forces Group didn't use rifle protective armor until the TG Faust and PTOA vests in 1986-1987, because non-ceramic rifle armor hindered their movement too much due to weight -- remember, their budget is and was nearly limitless. There's pictures of Delta guys during Eagle Claw wearing LEAA-cut soft armor carriers in 1979, but until the mid-1990s the SOF community largely lagged behind Law Enforcement in armor usage, particularly with rifle protection. The first big SOF program to procure an SOF-specific armor system is Ranger Body Armor, which was designed by running a contest of all the available carriers the Rangers wanted to buy, then taking all the best ideas from those. There was much consternation about the plates for RBA -- they were given the choice of steel or ceramics, but monolithic ceramic plates of the required ballistic proof were prohibitively expensive at the time -- something like $200/plate versus $50/plate for steel. Ultimately, someone pitched a Ceramic Tile Array plate that they were able to make for about $100/plate. I'm not sure why the backplate was omitted, the individual on the program that I spoke to said it was because the Rangers didn't feel they needed them because Rangers Lead The Way, but it may also have been a cost cutting measure.

Then, Mogadishu happens.

In the 1993 Battle of Mogadishu, you had Rangers come home with multiple bullets in their plate. You also had at least one person not come home because they didn't have a back plate. It is in many ways a defining moment in modern combat equipment, with the utility of rifle protective armor, widespread proliferation of night vision equipment, and low-power variable optics all proven in a very short period of time. All of these, however, were in their relative infancy. Armor was particularly desired, and particularly cost-prohibitive, not a winning combination during the defense draw-downs of the 1990s. There are a number of programs, including the excellent Personal Ballistic Protective Vest (which saw SOF use by 10th SFG and Delta despite being described as a "big army" program by persons who worked on the program), the SPEAR/BALCS program, Body Armor Vest Gen.II, and Interceptor. Interceptor, run by the Marines, had won out, as far as large-scale issuance was concerned.

By 2000, the Interceptor Body Armor program had been adopted by both the Army and Marine Corps (SOF units excepted, of course), but the issuance of plates in a wide scale was only possible due to substantial research into the manufacturing processes of armor ceramics conducted by Simula, now a subsidiary of BAe Systems, resulting in the Small Arms Protective Insert -- the results of this research as far as I am aware are largely not publicly available, but they were able to get the price of a SAPI to $500 in 2002/$728 in 2020 despite having extremely high performance targets as far as ballistic proof and areal density.

The Variable Study and some aspects of the SAPI Manufacturing Technologies report are available on DTIC. More information on the Variable Body Armor can be found in the Natick Labs 1987 Variable Armor Fact Sheet, and some memos of a member of the program. The sources for most of the more recent topics discussed are predominantly interviews with individuals associated with the programs in question.

To add to what u/Jon_Beveryman and u/AlasdhairM have written about the merits and demerits of steel armour and other armour, there isn't anything particularly special about steels like AR500. Similar chromium-nickel steel alloys were often used for helmets in WW1 and WW2, at about the same hardness (Brinell hardness of 500, 52HRC, 550VPH). For example WW1 and WW2 German helmets, which used such alloys, could protect from pistol rounds short of .45ACP (and could stop .45ACP depending on the speed), at a thickness of about 1.2mm. A modern AR500 Level IIIA plate can stop most handgun rounds, including .44 Magnum, but is insufficient to reliably stop rifle rounds. Such a plate is 5mm thick, about the same as the Kelly Gang's armour (note that Ned Kelly's armour weighed 44kg, prohibitively heavy for most purposes).

A similar thickness was used for the WW1 German brow plate helmet reinforcement. Along with the helmet, this provided reasonable protection against rifle bullets at trench-to-trench range (but at the cost of an additional 2kg worn on the head).

As far as steel armour goes, the modern version suffers from the same problems as WW1 and WW2 versions: too much weight is required for effective protection against rifle bullets.

(The US M1 helmet and the British Brodie helmet used a quite different alloy, Hadfield steel, a high manganese (about 12%) alloy. This gave similar protection for similar weight.)

A nice study of the composition and ballistic protection of WW2 German helmets: "Metallurgical and Ballistic Investigation of Fifty Captured German Helmets", Watertown Arsenal Laboratory, Report No. WAL. 710/670, 1944: https://apps.dtic.mil/dtic/tr/fulltext/u2/a954454.pdf

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!