High Strength Low Alloy Steel

Carbon-Manganese Steels (CMn) are a lower cost approach to reach up to approximately 280MPa yield strength, but are limited in ductility, toughness and welding.

Increasing carbon and manganese, along with alloying with other elements like chromium and silicon, will increase strength, but have the same challenges as CMn steels with higher cost. An example is AISI/SAE 4130, a chromium-molybdenum (chromoly) medium carbon alloy steel. A wide range of properties are available, depending on the heat treatment of formed components. Welding conditions must be carefully controlled.

The 1980s saw the commercialization of high-strength low-alloy (HSLA) steels. In contrast with alloy steels, HSLA steels achieved higher strength with a much lower alloy content. Lower carbon content and lower alloying content leads to increased ductility, toughness, and weldability compared with grades achieving their strength from only solid solution strengthening like CMn steels or from alloying like AISI/SAE 4130. Lower alloying and elimination of post-forming heat treatment makes HSLA steels an economical approach for many applications.

This steelmaking approach allows for the production of sheet steels with yield strength levels now approaching 800 MPa. HSLA steels increase strength primarily by micro-alloying elements contributing to fine carbide precipitation, substitutional and interstitial strengthening, and grain-size refinement. HSLA steels are found in many body-in-white and underbody structural applications where strength is needed for increased in-service loads.

These steels may be referred to as microalloyed steels, since the carbide precipitation and grain-size refinement is achieved with only 0.05% to 0.10% of titanium, vanadium, and niobium, added alone or in combination with each other.

HSLA steels have a microstructure that is mostly precipitation-strengthened ferrite, with the amount of other constituents like pearlite and bainite being a function of the targeted strength level. More information about microstructural components is available here.

Some of the specifications describing uncoated cold rolled high strength low alloy (HSLA) steel are included below, with the grades typically listed in order of increasing minimum yield strength and ductility. Different specifications may exist which describe hot or cold rolled, uncoated or coated, or steels of different strengths. Many automakers have proprietary specifications which encompass their requirements.  Note that ASTM, EN and VDA terminology is based on minimum yield strength, while JIS and JFS standards are based on minimum tensile strength.  Also note that JIS G3135 does not explicitly state that these grades must be supplied with an HSLA chemistry.  A C-Mn approach is satisfactory as long as the mechanical property criteria are satisfied.

  • ASTM A1008M, with the terms HSLAS 45[310], 50[340], 55[380], 60[410], 65[450], and 70[480] along with HSLAS-F 50 [340], 60 [410], Grade 70 [480] and 80 [550]A-25
  • EN10268, with the terms HC260LA, HC300LA, HC340LA, HC380LA, HC420LA, HC460LA, and HC500LAD-5
  • JIS G3135, with the terms SPFC340, SPFC370, SPFC390, SPFC440, SPFC490, SPFC540, and SPFC590J-3
  • JFS A2001, with the terms JSC440R and JSC590RJ-23
  • VDA239-100, with the terms CR210LA, CR240LA, CR270LA, CR300LA, CR340LA, CR380LA, CR420LA, and CR460LAV-3

Interstitial-Free High Strength

ULC, IF, VD-IF, and EDDS are interchangeable terms that describe the most formable (high n-value) and lowest strength grade of steel.  Adding phosphorus, manganese, and/or silicon to these grades increases the strength due to solid solution strengthening, precipitation of carbides and/or nitrides, and grain refinement. 

For most alloys, steelmaking practices attempt to reduce phosphorus to very low levels, since increased phosphorus content is sometimes associated with an increased risk of embrittlement. However, in the ladle metallurgy station after steelmaking, small controlled amounts of phosphorus are added back to the melt when certain grades are produced, leading to the term “rephosphorized.”  Phosphorus is a potent solid solution strengthening element, where only small additions result in large increases in yield and tensile strength. 

When phosphorus or other solid solution strengthening elements are used to increase the strength of interstitial-free steels, IF-HS (Interstitial-Free High Strength) steel is produced. Using phosphorus leads to the term IF-Rephosphorized steel, or IF-Rephos.

These alloys have composition controlled to improve r-value.  In some products, small amounts of boron are added to counteract the embrittlement effects brought on by the phosphorus.

These higher strength IF-HS grades are widely used for both structural and closure applications.  Work hardening from forming increases panel strength, which is why they may be described as dent resistant steels.  However, this alloying approach is not capable of producing a bake hardenable grade.

Compared bake hardenable steels, carbon-manganese steels, and HSLA steels at similar strength levels, IF-HS grades are more formable, resulting from the ultra-low carbon chemistry and interstitial-free microstructure. 

Some of the specifications describing uncoated cold rolled interstitial-free high strength (IF-HS) steel are included below, with the grades typically listed in order of increasing minimum yield strength and ductility.  Different specifications may exist. Many automakers have proprietary specifications which encompass their requirements.  Note that EN and VDA terminology is based on minimum yield strength, while JFS standard is based on minimum tensile strength.

  • EN10268, with the terms HC180Y, HC220Y, and HC260Y D-17
  • VDA239-100, with the terms CR160IF, CR180IF, CR210IF, and CR240IF V-3
  • JFS A2001, with the terms JSC340P, JSC370P, JSC390P, and JSC440P J-23