B-93. E. Bellhouse and J. McDermid, “Effect of Continuous Galvanizing Heat Treatments on the Microstructure and Mechanical Properties of High Al-Low Si Transformation Induced Plasticity Steels,” Metallurgical And Materials Transactions A, Volume 41A, June 2010, page 1460–1473; https://doi.org/10.1007/s11661-010-0185-7
Z-20. X. Zhao, Y. Zhang , C. Shao, W. Hui, and H. Dong, “Thermal stability of retained austenite and mechanical properties of medium-Mn steel during tempering treatment,” Journal of Iron and Steel Research International, 2017, 24(8): 830-837; http://www.chinamet.cn/gyby/gtyjxben/EN/Y2017/V24/I8/830
The microstructure of Transformation Induced Plasticity (TRIP) steels contains a matrix of ferrite, with retained austenite, martensite, and bainite present in varying amounts. Production of TRIP steels typically requires the use of an isothermal hold at an intermediate temperature, which produces some bainite. Higher silicon and carbon content of TRIP steels result in significant volume fractions of retained austenite in the final microstructure. Figure 1 shows a schematic of TRIP steel microstructure, with Figure 2 showing a micrograph of an actual sample of TRIP steel. Figure 3 compares the engineering stress-strain curve for HSLA steel to a TRIP steel curve of similar yield strength.
Figure 1: Schematic of a TRIP steel microstructure showing a matrix of ferrite, with martensite, bainite and retained austenite as the additional phases.
Figure 2: Micrograph of Transformation Induced Plasticity steel.
Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and TRIP 350/600.K-1
During deformation, the dispersion of hard second phases in soft ferrite creates a high work hardening rate, as observed in the DP steels. However, in TRIP steels the retained austenite also progressively transforms to martensite with increasing strain, thereby increasing the work hardening rate at higher strain levels. This is known as the TRIP Effect. This is illustrated in Figure 4, which compares the engineering stress-strain behavior of HSLA, DP and TRIP steels of nominally the same yield strength. The TRIP steel has a lower initial work hardening rate than the DP steel, but the hardening rate persists at higher strains where work hardening of the DP begins to diminish. Additional engineering and true stress-strain curves for TRIP steel grades are shown in Figure 5.
Figure 4: TRIP 350/600 with a greater total elongation than DP 350/600 and HSLA 350/450. K-1
Figure 5: Engineering stress-strain (left graphic) and true stress-strain (right graphic) curves for a series of TRIP steel grades. Sheet thickness: TRIP 350/600 = 1.2mm, TRIP 450/700 = 1.5mm, TRIP 500/750 = 2.0mm, and Mild Steel = approx. 1.9mm. V-1
The strain hardening response of TRIP steels are substantially higher than for conventional HSS, resulting in significantly improved formability in stretch deformation. This response is indicated by a comparison of the n-value for the grades. The improvement in stretch formability is particularly useful when designers take advantage of the improved strain hardening response to design a part utilizing the as-formed mechanical properties. High n-value persists to higher strains in TRIP steels, providing a slight advantage over DP in the most severe stretch forming applications.
Austenite is a higher temperature phase and is not stable at room temperature under equilibrium conditions. Along with a specific thermal cycle, carbon content greater than that used in DP steels are needed in TRIP steels to promote room-temperature stabilization of austenite. Retained austenite is the term given to the austenitic phase that is stable at room temperature.
Higher contents of silicon and/or aluminum accelerate the ferrite/bainite formation. These elements assist in maintaining the necessary carbon content within the retained austenite. Suppressing the carbide precipitation during bainitic transformation appears to be crucial for TRIP steels. Silicon and aluminum are used to avoid carbide precipitation in the bainite region.
The carbon level of the TRIP alloy alters the strain level at which the TRIP Effect occurs. The strain level at which retained austenite begins to transform to martensite is controlled by adjusting the carbon content. At lower carbon levels, retained austenite begins to transform almost immediately upon deformation, increasing the work hardening rate and formability during the stamping process. At higher carbon contents, retained austenite is more stable and begins to transform only at strain levels beyond those produced during forming. At these carbon levels, retained austenite transforms to martensite during subsequent deformation, such as a crash event.
TRIP steels therefore can be engineered to provide excellent formability for manufacturing complex AHSS parts or to exhibit high strain hardening during crash deformation resulting in excellent crash energy absorption.
The additional alloying requirements of TRIP steels degrade their resistance spot-welding behavior. This can be addressed through weld cycle modification, such as the use of pulsating welding or dilution welding. Table 1 provides a list of current production grades of TRIP steels and example automotive applications:
Table 1: Current Production Grades Of TRIP Steels And Example Automotive Applications.
Some of the specifications describing uncoated cold rolled 1st Generation transformation induced plasticity (TRIP) 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 Transformation induced plasticity (TRIP) steel Grades 690T/410Y and 780T/440YA-22
• JFS A2001, with the terms JSC590T and JSC780TJ-23
• EN 10338, with the terms HCT690T and HCT780TD-18
• VDA 239-100, with the terms CR400Y690T-TR and CR450Y780T-TRV-3
• SAE J2745, with terms Transformation Induced Plasticity (TRIP) 590T/380Y, 690T/400Y, and 780T/420YS-18
Austenite is not stable at room temperature under equilibrium conditions. An austenitic microstructure is retained at room temperature with the combined use of a specific chemistry and controlled thermal cycle.
Deformation from sheet forming or from crash impact provides the necessary energy to allow the crystallographic structure to change from austenite to martensite. There is insufficient time and temperature for substantial diffusion of carbon to occur from carbon-rich austenite, which results in a high-carbon (high strength) martensite after transformation. Strengthening also occurs from the dislocations formed in the adjacent ferrite required to accommodate the volume increase associated with the austenite-to-martensite transformation.
Transformation to high strength martensite continues as deformation increases, as long as retained austenite (RA) is still available to be transformed. Optimal combinations of strength and ductility are obtained when the retained austenite stability is such that the transformation to martensite occurs gradually with increasing strain.
Alloys capable of the TRIP effect are characterized by a high ductility – high strength combination. Such alloys include 1st Gen AHSS TRIP steels, as well as several 3rd Gen AHSS grades like TRIP-Assisted Bainitic Ferrite, Carbide Free Bainite, and Quench & Partition Steels.
In these grades, increasing the stability of the retained austenite phase delays the austenite-to-martensite transformation to higher strain levels, further promoting formability improvements.
Several factors may promote higher RA stability, including additions of carbon (C) and manganese (Mn). Smaller austenite grains lower the martensite start temperature (Ms) and the number of martensite nucleation sites in each grain, and as such more energy (strain) is needed to start the transformation.
Additions of silicon (Si), chromium (Cr), and aluminum (Al) are also beneficial to achieving the TRIP effect since each of these elements suppress cementite (iron carbide, Fe3C) formation and thereby allows for carbon enrichment of austenite.
However, Mn, Si, Cr, and Al all form oxides on the steel surface that hinder galvanizing and paintability associated with the e-coat layer. Steelmakers typically choose an alloy development and processing strategy which minimizes the detrimental effects of these oxides.
Temperature also has an effect, not only from the paint-bake temperatures of approximately 170 °C, but from galvanizing at close to 500 °C. Citation Z-20 studied the effects of temperature on 0.1%C-5%Mn Medium Manganese steels and found that while tensile strength was relatively independent with temperature, ductility slightly decreases as the temperature is raised from room temperature to 400 °C, but drops off substantially by 500 °C. To retain the formability benefits associated with RA grades, the article recommends galvanizing at temperatures below 400 °C.
In addition to the paint-bake and galvanizing temperatures, adiabatic heating from forming (including shearing and stamping) impact properties. The temperature during forming can be influenced by the starting ambient temperature, the plastic energy dissipation, the latent heat of transformation and by conduction and convection to the environment.M-76
While deforming a metal, most of the energy is dissipated in the form of heat while only a small amount is stored. Austenite-to-martensite transformation kinetics are highly influenced by temperature, and the heating effects associated with mechanically-induced transformation can lead to a severe reduction in ductility.
The temperature rise due to dissipation is not negligible and since the TRIP effect is extremely sensitive to temperature, there is a need for a model to predict this behavior well. Such a model is described in Citation M-76, which reviews that retained austenite stability is a function of several parameters such as temperature; carbon content and alloying elements; austenite grain size and morphology; austenite grain orientation and distribution within the microstructure; and hydrostatic pressure.
Complex Phase (CP) steels combine high strength with relatively high ductility. The microstructure of CP steels contains small amounts of martensite, retained austenite and pearlite within a ferrite/bainite matrix. A thermal cycle that retards recrystallization and promotes Titanium (Ti), Vanadium (V), or Niobium (Nb) carbo-nitrides precipitation results in extreme grain refinement. Minimizing retained austenite helps improve local formability, since forming steels with retained austenite induces the TRIP effect producing hard martensite.F-11
The balance of phases, and therefore the properties, results from the thermal cycle, which itself is a function of whether the product is hot rolled, cold rolled, or produced using a hot dip process. Citation P-18 indicates that galvannealed CP steels are characterized by low yield value and high ductility, whereas cold rolled CP steels are characterized by high yield value and good bendability. Typically these approaches require different melt chemistry, potentially resulting in different welding behavior.
CP steel microstructure is shown schematically in Figure 1, with the grain structure for hot rolled CP 800/1000 shown in Figure 2. The engineering stress-strain curves for mild steel, HSLA steel, and CP 1000/1200 steel are compared in Figure 3.
Figure 1: Schematic of a complex phase steel microstructure showing martensite and retained austenite in a ferrite-bainite matrix.
Figure 2: Micrograph of complex phase steel, HR800Y980T-CP.C-14
Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and CP 1000/1200.
DP and TRIP steels do not rely on precipitation hardening for strengthening, and as a result, the ferrite in these steels is relatively soft and ductile. In CP steels, carbo-nitride precipitation increases the ferrite strength. For this reason, CP steels show significantly higher yield strengths than DP steels at equal tensile strengths of 800 MPa and greater. Engineering and true stress-strain curves for CP steel grades are shown in Figure 4.
Figure 4: Engineering stress-strain (left graphic) and true stress-strain (right graphic) curves for a series of CP steel grades. Sheet thickness: CP650/850 = 1.5mm, CP 800/1000 = 0.8mm, CP 1000/1200 = 1.0mm, and Mild Steel = approx. 1.9mm.V-1
Whereas Dual Phase and TRIP steels excel at stretch forming and deep drawing, they are not as good in bending and cut-edge stretching. For applications needing these characteristics, complex phase steels are a good choice.
Table 1 compares galvannealed Dual Phase and Complex Phase steels having similar tensile strength and elongation as measured in a tensile test. The characteristics with the biggest difference are the yield strength and the hole expansion ratio which is a measure of cut-edge stretchability.
In the VDA-238 bending test, 980 DP exhibited a significant fracture with a bending angle of 67°. The 980 CP showed no fracture at 80°, and was capable of bending to over 100° before fracturing, Figure 5.N-33 The galvannealed 980 CP suppressed crack generation in axial crushing deformation to a much greater degree than the 980 DP.
Figure 5. Bendability of 980 DP (sample A) and 980 CP (sample B).N-33
Examples of typical automotive applications benefitting from these high strength steels with good local formability include frame rails, frame rail and pillar reinforcements, transverse beams, fender and bumper beams, rocker panels, and tunnel stiffeners.
Some of the specifications describing uncoated cold rolled 1st Generation complex phase (CP) 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.
Steel grades are engineered to achieve specific properties and characteristics by the manipulation of mill processing parameters to achieve a targeted balance of microstructural components. Among the tools available to the steelmaker are alloy composition, rolling and processing temperatures, and cooling profile.
If steel is slowly cooled, only two components exist at room temperature: ferrite (abbreviated by the Greek letter α) and cementite (iron carbide, Fe3C). Alternating layers of ferrite and cementite appear under a microscope in a pattern similar to Mother-of-Pearl, leading to the term pearlite.
A steel alloy having approximately 0.80% carbon will contain only pearlite in the microstructure. Lower carbon levels create an alloy that combines ferrite and pearlite. Ferrite-pearlite microstructures form the basis of many C-Mn steels and some of the initial HSLA steels. At a given strength level, pearlite limits sheet formability.
At carbon levels below 0.008% or 80 ppm, only ferrite exists. Ferrite is low strength but very ductile, and is the microstructural phase in ultra-low carbon steels.
Additional phases are formed when the cooling profile can be changed. Some modern annealing furnaces are capable of controlling the cooling rate as well as holding at specific temperatures. This ability is a key facilitator in the production of most Advanced High Strength Steels. In addition to ferrite and pearlite, microstructural phases of bainite, austenite, and martensite can be produced, depending on the chemistry and the thermal cycle profile including quench rate and hold temperature.
Bainite is a phase that is associated with enhanced sheared edge ductility. Accelerated cooling in the hot mill run out table allows for the production of Ferrite-Bainite steels.
Austenite is not stable at room temperature under equilibrium conditions. An austenitic microstructure is retained at room temperature with the use of a combined chemistry and controlled thermal cycle. Deforming retained austenite is responsible for the TRIP effect.
Martensite is a very high strength phase, but has limited toughness. Steels with both ferrite and martensite in the microstructure are known as dual phase steels. A structure of 100% martensite can be produced directly at those sheet mills having equipment capable of achieving a minimum critical cooling rate. The ductility of this product is not sufficiently high for most stamping operations. However, sheet martensite is well suited for properly designed roll forming applications.
Martensite is the microstructural component of processed Press Hardening Steels. An elevated temperature and a more formable microstructure exist at the time of complex forming of these grades. Rapid cooling while the part is under full press load converts the microstructure to the high strength martensite.