Dual Phase (DP) steels have a microstructure consisting of a ferritic matrix with martensitic islands as a hard second phase, shown schematically in Figure 1. The soft ferrite phase is generally continuous, giving these steels excellent ductility. When these steels deform, strain is concentrated in the lower-strength ferrite phase surrounding the islands of martensite, creating the unique high initial work-hardening rate (n-value) exhibited by these steels. Figure 2 is a micrograph showing the ferrite and martensite constituents.
Figure 1: Schematic of a Dual Phase steel microstructure showing islands of martensite in a matrix of ferrite.
Figure 2: Micrograph of Dual Phase Steel
Hot rolled DP steels do not have the benefit of an annealing cycle, so the dual phase microstructure must be achieved by controlled cooling from the austenite phase after exiting the hot strip mill finishing stands and before coiling. This typically requires a more highly alloyed chemistry than cold rolled DP steels require. Higher alloying is generally associated with a change in welding practices.
Continuously annealed cold-rolled and hot-dip coated Dual Phase steels are produced by controlled cooling from the two-phase ferrite plus austenite (α + γ) region to transform some austenite to ferrite before a rapid cooling transforms the remaining austenite to martensite. Due to the production process, small amounts of other phases (bainite and retained austenite) may be present.
Higher strength dual phase steels are typically achieved by increasing the martensite volume fraction. Depending on the composition and process route, steels requiring enhanced capability to resist cracking on a stretched edge (as typically measured by hole expansion capacity) can have a microstructure containing significant quantities of bainite.
The work hardening rate plus excellent elongation creates DP steels with much higher ultimate tensile strengths than conventional steels of similar yield strength. Figure 3 compares the engineering stress-strain curve for HSLA steel to a DP steel curve of similar yield strength. The DP steel exhibits higher initial work hardening rate, higher ultimate tensile strength, and lower YS/TS ratio than the HSLA with comparable yield strength. Additional engineering and true stress-strain curves for DP steel grades are presented in Figures 4 and 5.
Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and DP 350/600K-1
Figure 4: Engineering stress-strain curves for a series of DP steel grades.S-5, V-1 Sheet thicknesses: DP 250/450 and DP 500/800 = 1.0mm. All other steels were 1.8-2.0mm.
Figure 5: True stress-strain curves for a series of DP steel grades.S-5, V-1 Sheet thicknesses: DP 250/450 and DP 500/800 = 1.0mm. All other steels were 1.8-2.0mm.
DP and other AHSS also have a bake hardening effect that is an important benefit compared to conventional higher strength steels. The extent of the bake hardening effect in AHSS depends on an adequate amount of forming strain for the specific chemistry and thermal history of the steel.
In DP steels, carbon enables the formation of martensite at practical cooling rates by increasing the hardenability of the steel. Manganese, chromium, molybdenum, vanadium, and nickel, added individually or in combination, also help increase hardenability. Carbon also strengthens the martensite as a ferrite solute strengthener, as do silicon and phosphorus. These additions are carefully balanced, not only to produce unique mechanical properties, but also to maintain the generally good resistance spot welding capability. However, when welding the higher strength grades (DP 700/1000 and above) to themselves, the spot weldability may require adjustments to the welding practice.
Examples of current production grades of DP steels and typical automotive applications include:
|Roof outer, door outer, body side outer, package tray, floor panel
|Floor panel, hood outer, body side outer, cowl, fender, floor reinforcements
|Body side inner, quarter panel inner, rear rails, rear shock reinforcements
|Safety cage components (B-pillar, floor panel tunnel, engine cradle, front sub-frame package tray, shotgun, seat)
Some of the specifications describing uncoated cold rolled 1st Generation dual phase (DP) steel are included below, with the grades typically listed in order of increasing minimum tensile 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.
- ASTM A1088, with the terms Dual phase (DP) steel Grades 440T/250Y, 490T/290Y, 590T/340Y, 780T/420Y, and 980T/550YA-22
- EN 10338, with the terms HCT450X, HCT490X, HCT590X, HCT780X, HCT980X, HCT980XG, and HCT1180XD-6
- JIS G3135, with the terms SPFC490Y, SPFC540Y, SPFC590Y, SPFC780Y and SPFC980YJ-3
- JFS A2001, with the terms JSC590Y, JSC780Y, JSC980Y, JSC980YL, JSC980YH, JSC1180Y, JSC1180YL, and JSC1180YHJ-23
- VDA 239-100, with the terms CR290Y490T-DP, CR330Y590T-DP, CR440Y780T-DP, CR590Y980T-DP, and CR700Y980T-DPV-3
- SAE J2745, with terms Dual Phase (DP) 440T/250Y, 490T/290Y, 590T/340Y, 6907/550Y, 780T/420Y, and 980T/550YS-18
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 , BHS 31 , BHS 35 , BHS 41 , BHS 44 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. 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.
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.
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. 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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