Conventional HSS, Steel Grades
Carbon-Manganese High Strength Steel
Carbon and manganese are the two most cost-effective alloying additions to increase strength. While effective at strengthening, these additions reduce ductility and toughness, and make welding more challenging.
The practical usage of these grades typically limits the highest strength to no more than 280 MPa. Adding enough carbon and manganese to achieve higher strength results in a product without sufficient ductility for challenging applications, low toughness, and welding difficulty. These products sometimes are referred to as structural steels, and achieve their strength from the mechanism of solid solution strengthening.
Until the commercialization of High Strength Low Alloy steels, the CMn approach was the only option for users to obtain a high strength sheet metal.
Some of the specifications describing uncoated cold rolled Carbon-Manganese (CMn) or structural steels 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 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 a C-Mn chemistry. An HSLA approach is satisfactory as long as the mechanical property criteria are satisfied.
- ASTM A1008M, with the terms Grade 25 [170], Grade 30 [205], Grade 33 [230] Type 1, Grade 33 [230] Type 2, Grade 40 [275] Type 1, Grade 40 [275] Type 2, Grade 45 [310], Grade 50 [340], Grade 60 [410], Grade 70 [480], and Grade 80 [550] A-25
- JIS G3135 with the terms SPFC340, SPFC370, SPFC390, SPFC440, SPFC490, SPFC540, and SPFC590 J-3
- JFS A2001, with the terms JSC340W, JSC370W, JSC390W, and JSC440W J-23
Conventional HSS, Steel Grades
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
