High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather specific mechanical properties. They have a carbon content between 0.05 and 0.25% to retain formability and weldability. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare-earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added for strengthening purposes.
HSLA steels are also more resistant to rust than most carbon steels because of their lack of pearlite – the fine layers of ferrite (almost pure iron) and cementite in pearlite. HSLA steels usually have densities of around 7800 kg/m<sup>3</sup>.
Military armour plate is mostly made from alloy steels, although some civilian armour against small arms is now made from HSLA steels with extreme low temperature quenching.
SAE grades
The Society of Automotive Engineers (SAE) maintains standards for HSLA steel grades because they are often used in automotive applications.
{| class="wikitable" border="1" style="margin-left:auto;margin-right:auto;"
|+ SAE HSLA steel grade compositions
|-
! Grade !! % Carbon (max) !! % Manganese (max) !! % Phosphorus (max) !! % Sulfur (max) !! % Silicon (max) !! Notes
|-
| 942X || 0.21 || 1.35 || 0.04 || 0.05 || 0.90 || Niobium or vanadium treated
|-
| 945A || 0.15 || 1.00 || 0.04 || 0.05 || 0.90 ||
|-
| 945C || 0.23 || 1.40 || 0.04 || 0.05 || 0.90 ||
|-
| 945X || 0.22 || 1.35 || 0.04 || 0.05 || 0.90 || Niobium or vanadium treated
|-
| 950A || 0.15 || 1.30 || 0.04 || 0.05 || 0.90 ||
|-
| 950B || 0.22 || 1.30 || 0.04 || 0.05 || 0.90 ||
|-
| 950C || 0.25 || 1.60 || 0.04 || 0.05 || 0.90 ||
|-
| 950D || 0.15 || 1.00 || 0.15 || 0.05 || 0.90 ||
|-
| 950X || 0.23 || 1.35 || 0.04 || 0.05 || 0.90 || Niobium or vanadium treated
|-
| 955X || 0.25 || 1.35 || 0.04 || 0.05 || 0.90 || Niobium, vanadium, or nitrogen treated
|-
| 960X || 0.26 || 1.45 || 0.04 || 0.05 || 0.90 || Niobium, vanadium, or nitrogen treated
|-
| 965X || 0.26 || 1.45 || 0.04 || 0.05 || 0.90 || Niobium, vanadium, or nitrogen treated
|-
| 970X || 0.26 || 1.65 || 0.04 || 0.05 || 0.90 || Niobium, vanadium, or nitrogen treated
|-
| 980X || 0.26 || 1.65 || 0.04 || 0.05 || 0.90 || Niobium, vanadium, or nitrogen treated
|}
{| class="wikitable" border="1" style="margin-left:auto;margin-right:auto;"
|+ SAE HSLA steel grade mechanical properties
|-
! Grade !! Form !! Yield strength (min) [psi (MPa)] !! Ultimate tensile strength (min) [psi (MPa)]
|-
| 942X || Plates, shapes & bars up to 4 in. || ||
|-
| rowspan="5" | 945A, C || Sheet & strip || ||
|-
| Plates, shapes & bars: || ||
|-
| align="right" | 0–0.5 in. || ||
|-
| align="right" | 0.5–1.5 in. || ||
|-
| align="right" | 1.5–3 in. || ||
|-
| 945X || Sheet, strip, plates, shapes & bars up to 1.5 in. || ||
|-
| rowspan="5" | 950A, B, C, D || Sheet & strip || ||
|-
| Plates, shapes & bars: || ||
|-
| align="right" | 0–0.5 in. || ||
|-
| align="right" | 0.5–1.5 in. || ||
|-
| align="right" | 1.5–3 in. || ||
|-
| 950X || Sheet, strip, plates, shapes & bars up to 1.5 in. || ||
|-
| 955X || Sheet, strip, plates, shapes & bars up to 1.5 in. || ||
|-
| 960X || Sheet, strip, plates, shapes & bars up to 1.5 in. || ||
|-
| 965X || Sheet, strip, plates, shapes & bars up to 0.75 in. || ||
|-
| 970X || Sheet, strip, plates, shapes & bars up to 0.75 in. || ||
|-
| 980X || Sheet, strip & plates up to 0.375 in. || ||
|}
{| class="wikitable" border="1" style="margin-left:auto;margin-right:auto;"
|+ Ranking of various properties for SAE HSLA steel grades
|-
! Rank !! Weldability !! Formability !! Toughness
|-
| Worst || 980X || 980X || 980X
|-
| rowspan="9" | ||| 970X || 970X || 970X
|-
| 965X || 965X || 965X
|-
| 960X || 960X || 960X
|-
| 955X, 950C, 942X || 955X || 955X
|-
| 945C || 950C || 945C, 950C, 942X
|-
| 950B, 950X || 950D || 945X, 950X
|-
| 945X || 950B, 950X, 942X || 950D
|-
| 950D || 945C, 945X || 950B
|-
| 950A || 950A || 950A
|-
| Best || 945A || 945A || 945A
|}
Controlled rolling of HSLA steels
Mechanism
Controlled rolling
thumb|Change in microstructure at different controlled-rolling stages.
Controlled rolling is a method of refining the grain of steel by introducing a large amount of nucleation sites for ferrite in the austenite matrix by rolling it at precisely controlled temperature, thereby increasing the strength of the steel. There are three main stages in controlled rolling:
1) Deformation in recrystallization regions. In this stage, austenite is being recrystallized and refined, enabling refinement of ferrite grains in a later stage.
2) Deformation in non-recrystallization regions. Austenite grains are elongated by rolling. Deformation bands might present within the band as well. Elongated grain boundaries and deformation bands are all nucleation sites for ferrite.
3) Deformation in austenite-ferrite two phase region. Ferrite nucleates and austenite are further work-hardened.
Strengthening Mechanism
Control-rolled HSLA steels contain a combination of different strengthening mechanisms. The main strengthening effect comes from grain refinement (Grain boundary strengthening), in which strength increases as the grain size decreases. The other mechanisms include solid solution strengthening and precipitate hardening from micro-alloyed elements. After the steel passes the temperature of austenite-ferrite region, it is then further strengthened by work hardening.
Sources
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