let words = String.Join(" ", Array.map (fun (s : String) -> (String.capitalize (s.ToLower()))) ("man OF stEEL".Split [|' '|]))

#light
open System

let words = String.Join(" ", Array.map (fun (s : String) -> (String.capitalize (s.ToLower()))) ("man OF stEEL".Split [|' '|]))

printfn "%s" words

let wordlst = List.map (fun s -> (String.capitalize (String.lowercase s))) (String.split [' '] "man OF stEEL")
let words = new StringBuilder(List.hd wordlst)
for (s : String) in (List.tl wordlst) do (words.Append(" ").Append(s))

#light
open System
open System.Text

let wordlst = List.map (fun s -> (String.capitalize (String.lowercase s))) (String.split [' '] "man OF stEEL")
let words = new StringBuilder(List.hd wordlst)
for (s : String) in (List.tl wordlst) do (words.Append(" ").Append(s))

printfn "%s" (words.ToString())

// Previous solutions used old library functions, here's something that works with F# 2.0
let s= "man OF stEEL"
let UpperFirst = function | "" -> "" | s -> s.Substring(0,1).ToUpper() + s.Substring(1).ToLower()
s.Split(' ') |> Array.map UpperFirst |> String.concat " "

// Previous solutions used old library functions, here's something that works with F# 2.0
let s= "man OF stEEL"
let UpperFirst = function | "" -> "" | s -> s.Substring(0,1).ToUpper() + s.Substring(1).ToLower()
s.Split(' ') |> Array.map UpperFirst |> String.concat " "

let culture = System.Globalization.CultureInfo.GetCultureInfo("en-US")
let titleCase = culture.TextInfo.ToTitleCase "man oF sTeel"

// Alternative: Retrieve current culture info from System
let currentCulture = System.Globalization.CultureInfo.CurrentCulture
let culture = System.Globalization.CultureInfo.GetCultureInfo("en-US")
let titleCase = culture.TextInfo.ToTitleCase "man oF sTeel"
do
printfn "%s" titleCase