The main factors responsible for increased strength in HSLA steels are given below:
Factor # 1. Grain-Size Refinement:
The very fine ferrite grain sizes in HSLA steels are possible by the control of austenite grain size by the precipitation of carbonitrides during hot rolling as the temperature of the steel falls. These fine precipitate particles hinder the growth of austenite grains, and at still lower temperatures of rolling, the particles inhibit even the recrystallisation of the deformed austenite grains.
Actually, the austenite grains may recrystallise several times during hot controlled rolling but the total effect of this will be a marked refinement in austenite grains by the time the steel reaches γ/α transformation temperature. In the later stages of austenite deformation at lower temperatures, recrystallisation may not occur.
The elongated and flattened deformed austenite grains may then transform directly to ferrite, or may be cooled rapidly from the finish rolling temperature so that γ/α transformations takes place subcritically to produce still finer ferrite grains.
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To accomplish this, the fine precipitation of carbonitrides should take place in the critical rolling range of 1300°C to 925°C, when the recrystallisation of austenite could occur, and that the volume of the precipitates formed should be large. It is thus, essential that these carbonitrides have sufficient solid solubility at the highest austenitising or soaking temperature and that the solid solubility should decrease fast with the fall of temperature in this critical range.
Ti N is the most stable of precipitates. It usually forms either during solidification, or during soaking at relatively high temperature (1200-1300°C). It is essential to use high soaking temperatures to dissolve as much of the elements niobium, titanium, vanadium, so that these could precipitate (as carbonitrides) later during rolling when the temperature continuously drops.
The presence of Ti-N precipitates restricts the grain growth of austenite at the soaking temperature and during the dynamic recrystallisation of austenite during hot rolling at high temperatures. The Zener and Pranjpe theory of boundary pinning by particles.
As fine a grain boundary precipitate (of TiN) as possible at the highest austenitising temperature which will not dissolve completely in austenite at the highest soaking temperature (1200- 1300°C) controls the grain growth of austenite. As nitrides are more stable than carbides, TiN is present normally at these temperatures.
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While grain growth at the highest austenitising temperature may be restricted to some extent by a residual dispersion of TiN, the main refinement is achieved during hot rolling from 1300° to 925°C as the temperature progressively falls and fine carbonitrides are precipitated from austenite.
These new precipitates increase the amount of plastic deformation needed at a temperature at which recrystallisation will take place (Fig. 9.10) and restrict the movement of recrystallised grain boundaries. Niobium is the most effective element in modifying the recrystallisation behaviour of austenite during hot rolling as niobium carbides and carbonitrides precipitate during hot rolling of austenite (800-1050°C), and hence is the most important micro-alloying element.
Fig. 9.11 illustrates its effectiveness in producing fine austenite grains. Also, the solubility of Nb (C, N) in austenite is less so that grain refinement can be obtained by its smaller additions. Another important aspect of adding Nb is that its precipitates produce grain refinement without recrystallisation even when rolling is finished at as high temperature as 925°C. Infact, the presence of niobium in steels does not allow recrystallisation to occur even at a high finishing temperature of 925°C.
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The carbides and carbonitrides of vanadium are seen to be generally precipitating at much lower temperatures either during the γ/α transformation as interphase precipitates or in ferrite itself. Thus, these precipitates mainly contribute to the strength of the steel by precipitation hardening.
Factor # 2. Controlled Rolling:
Controlled hot rolling of low carbon low alloy high strength steels is done to obtain ultra-fine, uniform grains of ferrite and precipitation hardening. Fig. 9.12 illustrates schematically grain size of austenite at different stages of hot rolling.
High temperature soaking is required to dissolve Nb, V, Ti in austenite so that these precipitate in fine dispersed state during rolling as carbides and carbonitrides. Having fine austenite grains, or very thin unrecrystallised austenite grains, just before austenite transforms to ferrite, is a pre-requisite for obtaining fine ferrite grains in such steels.
When austenite is rolled at relatively high temperatures, it dynamically recrystallises and the grain growth occurs. Presence of Ti N and precipitation of Nb (C, N) in this duration does some refining of grains.
The situation demands heavy deformation and low finishing temperatures in the austenitic region, below about 950°C, so that austenite is unable to recrystallise. Having the finishing temperature in partial recrystallisation range produces ferrite of duplex structure (i.e., regions of two different grain sizes) which is highly undesirable.
Thus heavy reductions, more than 50% is done in finishing stage in no-recrystallisation range of austenite to obtain very thin elongated and flattened grains of austenite. This finishing temperature is very important. Normally, all the deformation is done when the steel is austenitic.
In one practice of controlled rolling, the steel is continued to be rolled through the transformation into completely ferritic state. This helps to retain fine dislocation sub-grain structure in ferrite, strengthening sharply further the steel, but is accompanied by increase in ductile/brittle transition temperature due to this substructure in ferrite, and leads to splitting fracture.
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In another practice, the finishing temperature is above the γ/α transformation temperature, and the nature of the transformation is changed by increasing the cooling rate using water sprays following rolling. The subcritical transformation produces fine ferrite grains. This may induce widmanstatten ferrite with much higher dislocation density.
Mechanical properties are improved and the sharp yield point is invariably suppressed. Such steels are invaluable for pipe production which should not have sharp yield point.
Factor # 3. Precipitation Hardening:
Precipitation hardening also contributes to the increased strength of HSLA steels. The precipitates present or formed at high temperatures during controlled rolling cause little strengthening as they are large sized, widely spaced and as most of them are present at the grain boundaries controlling the grain growth.
The precipitate strengthening occurs by those particles that form:
(i) In austenite at low temperatures.
(ii) At the γ/α interface during transformation.
(iii) In ferrite during further cooling.
The main contribution to precipitation strengthening is the precipitation of carbides of niobium, titanium, vanadium which occurs during the transformation of austenite to ferrite progressively at interface boundaries-called interphase precipitation.
It occurs on very fine scale (size about 5 nm) during the temperature between 850° and 650°C. Because of high solubility in austenite, vanadium carbide and nitride precipitate at interphase boundaries and in ferrite, with titanium and niobium in the decreasing order, are most effective in increasing the strength by precipitation.
Solid solution strengthening due to presence of elements like manganese, silicon and even uncombined nitrogen takes place. The most effective factor is the ultra-fine grains of ferrite which could be of order of 5 µm in size in thin plates, though a size of 1-2 µm has been obtained under laboratory conditions.