Carbon-Manganese (CMn)

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

 

 

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

Mild Steels

Metallurgy of Mild Steels

Mild steels are low carbon steels with no alloying elements added for substantial strengthening, and for that reason are characterized by relatively lower yield strength. Typically, mild steels have less than 0.10% carbon.

These steels have a microstructure that is primarily ferrite. The amount of pearlite in the microstructure is a function of the amount of carbon in the steel, with lower carbon resulting in a lower fraction of pearlite. More information about microstructural components is available here.

Ultra-low carbon steels are a type of mild steel. These grades are typically the lowest yield strength and highest ductility available. Generally, these steels have less than 0.005% carbon, or less than 50 ppm C. More information is contained on the page for Ultra-Low Carbon steels.

Different names may describe mild steels, but these differences came from steel mill production techniques that are no longer in use. Since all sheet steels in use today are continuously cast, there are no significant differences between these terms:

  • Drawing Quality (DQ)
  • Drawing Steel (DS)
  • Aluminum Killed Drawing Quality (AKDQ)
  • Drawing Quality Aluminum Killed (DQAK)
  • Drawing Quality Special Killed (DQSK)

Mild steels are described in OEM, regional or global specifications with different syntax. While certain grade definitions between these specifications may be similar, the user is cautioned against using conversion charts without first confirming aspects which might be different, such as minimum, maximum, or typical values of chemical or mechanical properties. For example, ASTM specifications which cover lower strength steels list only typical values for tensile properties, but these are non-mandatory and the user may receive product outside the ranges shown. Additionally, JIS specifications which cover lower strength steels do not have minimum yield strength requirements, and the minimum elongation varies by thickness.

Some of the specifications describing uncoated cold rolled mild steel are included below, with the grades typically listed in order of increasing 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.

  • ASTM A1008M, with the terms CS, DS, DDS, and EDDSA-25
  • EN10130, with the terms DC01, DC03, DC04, DC05, and DC06D-3
  • JIS G3141, with the terms SPCC, SPCD, SPCE, SPCF, and SPCGJ-2
  • JFS A2001, with the terms JSC270C, JSC270D, JSC270E, JSC270F, and JSC260GJ-23
  • VDA239-100, with the terms CR1, CR2, CR3, CR4, and CR5V-3
Bake Hardenable

Bake Hardenable

BH Grades

Bake Hardenable (BH) steels grades are conventional High Strength Steels that exhibit a Bake Hardening Effect. BH steels exhibit an increase in yield strength after room-temperature stamping followed by processing through a thermal cycle comparable to the time-temperature profile used in paint curing (or baking) – approximately 170 °C for 20 minutes. Bake hardenability is characterized by determining the Bake Hardening Index.

Bake Hardenable steel grades have yield strength at shipment from the steel mills of 180 MPa to 300 MPa (approximately 25 ksi to 45 ksi). The grades at the lower strength levels are capable of being produced with a Class A surface finish and are used in applications where dent resistance is desired in thin sheet steel. Applications for the higher strength BH steels include structural parts where Class A surface is not required. The higher strength after forming and baking is the reason automakers might use these in body structure applications, potentially contributing to vehicle lightweighting efforts.

These grades work harden approximately 30 MPa when 2% strain is introduced, either from stamping or during a tensile test, which is similar to dent resistant IF-HS. In contrast to IF-HS, the paint-bake cycle after forming results in an additional yield strength increase. The minimum strength increase from baking is specified by some automakers as 20 MPa to 35 MPa, measured after applying a defined level of strain.

Higher yield strength directly improves the dent performance. Even though BH grades and their non bake hardening counterpart IF-HS grades may have similar yield strength and thickness after forming, bake hardenable steels will show superior dent resistance due to the increase in yield strength from the paint baking operation.

Ferrite is the main microstructural phase of BH steels. The strengthening from the paint bake cycle is due to the controlled amount of carbon remaining in solid solution (on the order of 25 ppm) when the steel leaves the production mill. At the baking temperatures after the part is formed, the dissolved carbon migrates to pin any free dislocations created from stamping. This increases the yield strength of the formed part for increased dent resistance. Formability does not suffer, since the strength increase occurs after stamping.

Most Advanced High Strength Steel (AHSS) grades also exhibit a Bake Hardening Effect, achieving yield strength increases of 40 MPa to 120 MPa from an appropriate thermal cycle. AHSS grades are not categorized with traditional bake hardenable steels, since their primary characteristics and applications are typically, but not exclusively, different. One exception are some Dual Phase (DP) steels available with a Class A surface, which are used as skin panels to combine excellent dent resistance with lightweighting benefits.

Some of the specifications describing uncoated Bake Hardenable (BH) steel are included below, with the grades typically listed in order of increasing minimum yield strength and ductility. Different specifications may exist which describe uncoated or coated, or steels of different strengths.

  • ASTM A1008M, with the terms BHS 26 [180], BHS 31 [210], BHS 35 [240], BHS 41 [280], BHS 44 [300]A-25
  • EN10268, with the terms HC180B, HC220B, HC260B, and HC300LAD-3
  • JIS G3135, with the term SPFC340HJ-3
  • JFS A2001, with the terms JSC270H, JSC340HJ-23
  • VDA239-100, with the terms CR180BH, CR210BH, CR240BH, and CR270BHV-3

 

Bake Hardening Effect

Bake Hardenable Steel Grades and most AHSS grades exhibit a Bake Hardening Effect, meaning that there is an increase in yield strength after room-temperature stamping followed by processing through a thermal cycle comparable to the time-temperature profile used in paint curing (or baking) – approximately 170 °C for 20 minutes.

The degree to which a sample is bake hardenable is characterized by the Bake Hardening Index.

In Bake Hardenable Steel Grades, solid solution hardening elements like phosphorus, manganese, and silicon are used to achieve the desired initial strength. For AHSS, the initial strength is determined by the balance and volume fraction of microstructural components like ferrite, bainite, retained austenite, and martensite. In both cases, a specifically engineered amount of dissolved carbon in the ferritic matrix causes an additional increase in the yield strength through controlled carbon aging during the paint-bake thermal cycle. The bake hardening process in AHSS grades is more complex, and results in substantially higher values of the Bake Hardening Index.

Figure 1 shows the work hardening and bake hardening increases for samples of three High-Strength steel grades having the same as-received yield strength prior to 2% pre-straining and baking. The HSLA steel shows little or no bake hardening, while AHSS such as DP and Transformation Induced Plasticity (TRIP) steels show a large positive bake hardening index. The DP steel also has significantly higher work hardening than HSLA or TRIP steel because of higher strain hardening at low strains. No aging behavior of AHSS has been observed due to storage of as-received coils or blanks over a significant length of time at normal room temperatures. Hence, significant mechanical property changes of shipped AHSS products during normal storage conditions are unlikely.

The higher bake hardening index (BHI) of AHSS grades DP 600 and TRIP 700 is also shown in Figure 2. While BHI is determined at a prestrain of 2%, this graph indicates that even higher levels of bake hardenability can be achieved with increasing strain. In a stamping where most areas have more than 2% strain, combining this higher bake hardenability with the increased work hardening that occurs with increasing strain results in a formed panel having a strength markedly higher than the incoming flat steel. This is beneficial for crash energy management.

Figure 1: Comparison of work hardening (WH) and bake hardening (BH) for TRIP, DP, and HSLA steels given a 2% prestrain. S1, K3

Figure 1: Comparison of work hardening (WH) and bake hardening (BH) for TRIP, DP, and HSLA steels given a 2% prestrain. S-1, K-3

 

Figure 2: Bake hardening responses of several HSS and AHSS products with varying pre-strain, reproduced from Figure 3 in Citation B-6. The bracketed numbers after each grade are references within the cited paper.

Figure 2: Bake hardening responses of several HSS and AHSS products with varying pre-strain, reproduced from Figure 3 in Citation B-6. The bracketed numbers after each grade are references within the cited paper.

 

Bake Hardenability of Exposed Quality Dual Phase Steels

Dent resistance is a function of the yield strength in the formed panel after it completes the paint baking cycle. Based on this premise, grades with higher bake hardenability, such as AHSS, should have substantially higher dent resistance. Application of AHSS grades to capitalize on improved dent resistance also requires their production at the desired thickness and width along with surface characteristics appropriate for Class A exposed quality panels. Some DP steels meet these tight requirements specified by the automotive industry.

A recent studyK-49 highlights this improved dent resistance. This work presents the experimental results and associated numerical investigation of the dent testing of DP270Y490T, a DP steel grade with 490 MPa minimum tensile strength. Tests performed to the SAE J2575 procedureS-7 measure the resultant dent depth after testing, so therefore smaller depths indicate improved performance. Compared with samples not processed through a bake hardening cycle, dent depth reductions occur with hotter and longer cycles, as shown in Figure 3. Increasing temperature plays a more significant role in dent depth reduction than increasing time. This work also reinforces that bake hardenability must be incorporated into simulation models in order to improve the accuracy of dent resistance predictions.

Figure 3: Dent resistance of DP270Y490T according to SAE J2575S-7* as a function of baking test conditions.K-49  Lower dent depth indicates better dent resistance.

Figure 3: Dent resistance of DP270Y490T according to SAE J2575S-7 as a function of baking test conditions.K-49 Lower dent depth indicates better dent resistance.

 

Measuring The Bake Hardenability Index

Bake hardenability is characterized by determining the Bake Hardening Index, or BHI.

The Bake Hardening Index (BH2) is determined by taking a conventional tensile test sample and pulling it to 2% strain. This is known as a 2% pre-strain. The sample is then put into an oven for a thermal cycle designed to be typical of an automotive paint curing (paint baking) cycle: 170 °C for 20 minutes. The temperature and time may be different depending on the end-user specifications.

Some companies may specify BH0, which uses the same thermal cycle without the 2% pre-strain. BH5 or BH10 (5% or 10% pre-strain, respectively) may also be reported.

The experimental procedure and calculation of BH2 is standardized in EN 10325D-4 and JIS G 3135J-3, and is similarly described in several other specifications.

Figure 4 defines the measurement for work hardening (B minus A), unloading to C for baking, and reloading to yielding at D for measurement of bake hardening (D minus B).  Note that the bake hardening index shown here is measured up to the lower yield point, which is consistent with the EN 10325 definition.  JIS G 3135 prescribes the use of the upper yield point.

Figure 4: Measurement of work hardening index and bake hardening index.

Figure 4: Measurement of work hardening index and bake hardening index.  BHI is measured using the lower yield point in EN 10325D-4 and with the upper yield point in JIS G 3135J-3.

 

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