T-57. O.R. Terrazas, K.O. Findley, C.J. Van Tyne, “Influence of Martensite Morphology on Sheared-Edge Formability of Dual-Phase Steels,” ISIJ International, 2017, Volume 57, Issue 5, Pages 937-944; https://www.jstage.jst.go.jp/article/isijinternational/57/5/57_ISIJINT-2016-602/_article; https://doi.org/10.2355/isijinternational.ISIJINT-2016-602.
J-27. D. Ji, C. Lian and F. Han, “Comparative research on the formability of several ultra-high-strength steels,” Baosteel Technical Research, Volume 17, Number 3, September 2023, Page 18; www.doi.org/10.3969/j.issn.1674-3458.2023.03.003
Automakers contemplating whether a part is cold stamped or hot formed must consider numerous ramifications impacting multiple departments. Over a series of blogs, we’ll cover some of the considerations that must enter the discussion.
The discussions relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid-formed.
There is a well-established infrastructure for cold stamping. New grades benefit from servo presses, especially for those grades where press force and press energy must be considered. Larger press beds may be necessary to accommodate larger parts. As long as these factors are considered, the existing infrastructure is likely sufficient.
Progressive-die presses have tonnage ratings commonly in the range of 630 to 1250 tons at relatively high stroke rates. Transfer presses, typically ranging from 800 to 2500 tons, operate at relatively lower stroke rates. Power requirements can vary between 75 kW (630 tons) to 350 kW (2500 tons). Recent transfer press installations of approximately 3000 tons capacity allow for processing of an expanded range of higher strength steels.
Hot stamping requires a high-tonnage servo-driven press (approximately 1000 ton force capacity) with a 3 meter by 2 meter bolster, fed by either a roller-hearth furnace more than 30 m long or a multi-chamber furnace. Press hardened steels need to be heated to 900 °C for full austenitization in order to achieve a uniform consistent phase, and this contributes to energy requirements often exceeding 2 MW.
Integrating multiple functions into fewer parts leads to part consolidation. Accommodating large laser-welded parts such as combined front and rear door rings expands the need for even wider furnaces, higher-tonnage presses, and larger bolster dimensions.
Blanking of coils used in the PHS process occurs before the hardening step, so forces are low. Post-hardening trimming usually requires laser cutting, or possibly mechanical cutting if some processing was done to soften the areas of interest.
That contrasts with the blanking and trimming of high strength cold-forming grades. Except for the highest strength cold forming grades, both blanking and trimming tonnage requirements are sufficiently low that conventional mechanical cutting is used on the vast majority of parts. Cut edge quality and uniformity greatly impact the edge stretchability that may lead to unexpected fracture.
Most cold stamped parts going into a given body-in-white are formed by a tier supplier. In contrast, some automakers create the vast majority of their hot stamped parts in-house, while others rely on their tier suppliers to provide hot stamped components. The number of qualified suppliers capable of producing hot stamped parts is markedly smaller than the number of cold stamping part suppliers.
Hot stamping is more complex than just adding heat to a cold stamping process. Suppliers of cold stamped parts are responsible for forming a dimensionally accurate part, assuming the steel supplier provides sheet metal with the required tensile properties achieved with a targeted microstructure.
Suppliers of hot stamped parts are also responsible for producing a dimensionally accurate part, but have additional responsibility for developing the microstructure and tensile properties of that part from a general steel chemistry typically described as 22MnB5.
Independent of which company creates the hot formed part, appropriate quality assurance practices must be in place. With cold stamped parts, steel is produced to meet the minimum requirements for that grade, so routine property testing of the formed part is usually not performed. This is in contrast to hot stamped parts, where the local quench rate has a direct effect on tensile properties after forming. If any portion of the part is not quenched faster than the critical cooling rate, the targeted mechanical properties will not be met and part performance can be compromised. Many companies have a standard practice of testing multiple areas on samples pulled every run. It’s critical that these tested areas are representative of the entire part. For example, on the top of a hat-section profile where there is good contact between the punch and cavity, heat extraction is likely uniform and consistent. However, on the vertical sidewalls, getting sufficient contact between the sheet metal and the tooling is more challenging. As a result, the reduced heat extraction may limit the strengthening effect due to an insufficient quench rate.
For more information, see our Press Hardened Steel Primer to learn more about PHS grades and processing!
Thanks are given to Eren Billur, Ph.D., Billur MetalForm for his contributions to the Equipment section, as well as many of the webpages relating to Press Hardening Steels at www.AHSSinsights.org.
Danny Schaeffler is the Metallurgy and Forming Technical Editor of the AHSS Applications Guidelines available from WorldAutoSteel. He is founder and President of Engineering Quality Solutions (EQS). Danny wrote the monthly “Science of Forming” and “Metal Matters” column for Metalforming Magazine, and provides seminars on sheet metal formability for Auto/Steel Partnership and the Precision Metalforming Association. He has written for Stamping Journal and The Fabricator, and has lectured at FabTech. Danny is passionate about training new and experienced employees at manufacturing companies about how sheet metal properties impact their forming success.
Our colleagues at JFE Steel recently provided us with a new case study based on laboratory evaluations they conducted in Japan. The article is part of our Martensite article, but we this month, we want to highlight it in our AHSS Insights blog.
Martensitic steel grades provide a cold formed alternative to hot formed press hardening steels. Not all product shapes can be cold formed. For those shapes where forming at ambient temperatures is possible, design and process strategies must address the springback which comes with the high strength levels, as well as eliminate the risk of delayed fracture. The potential benefits associated with cold forming include lower energy costs, reduced carbon footprint, and improved cycle times compared with hot forming processes.
Highlighting product forms achievable in cold stamping, an automotive steel Product Applications Laboratory formed a Roof Center Reinforcement from 1.4 mm CR1200Y1470T-MS using conventional cold stamping rather than roll forming, Figure 6. Using cold stamping allows for the flexibility of considering different strategies when die processing which may result in reduced springback or incorporating part features not achievable with roll forming.
Figure 6: Roof Center Reinforcement cold stamped from CR1200Y1470T-MS martensitic steel.U-1
Cold stamping of martensitic steels is not limited to simpler shapes with gentle curvature. Shown in Figure 7 is a Center Pillar Outer, cold stamped using a tailor welded blank containing CR1200Y1470T-MS and CR320Y590T-DP as the upper and lower portion steels.U-1
Figure 7: Center Pillar Outer stamped at ambient temperature from a tailor welded blank containing 1470 MPa tensile strength martensitic steel.U-1
Another characteristic of martensitic steels is their high yield strength, which is associated with improved crash performance. In a laboratory environment, crash behavior is assessed with 3-point bending moments. A studyS-8 determined there was a correlation between sheet steel yield strength and the 3-point bending deformation of hat shaped parts. Based on a comparison of yield strength, Figure 8 shows that CR1200Y1470T-MS has similar performance to hot stamped PHS-CR1800T-MB and PHS-CR1900T-MB at the same thickness and exceeds the frequently used PHS-CR1500T-MB. For this reason, there may be the potential to reduce costs and even weight with a cold stamping approach, providing appropriate press, process, and die designs are used.
Figure 8: Effect of Yield Strength on Bending Moment. The right image shows the typical yield strength range of CR1030Y1300T-MS and CR1200Y1470T-MS as well as typical yield strength values of several Press Hardened Steels.S-8
To read more about Martensitic steels, including its practical applications, visit the Steel Grades page here.
Many thanks to Toshiaki Urabe, Principal Researcher, JFE Steel, and Dr. Daniel Schaeffler, President, Engineering Quality Solutions, Inc., for providing this case study.
Martensitic steels are characterized by a microstructure that is mostly all martensite, but possibly also containing small amounts of ferrite and/or bainite (Figure 1 and 2). Steels with a fully martensitic microstructure are associated with the highest tensile strength – grades with a tensile strength of 2000 MPa is commercially available, and higher strength levels are under development.
Figure 1: Schematic of a martensitic steel microstructure. Ferrite and bainite may also be found in small amounts.
Figure 2: Microstructure of MS 950/1200
To create MS steels, the austenite that exists during hot-rolling or annealing is transformed almost entirely to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line. Adding carbon to MS steels increases hardenability and strengthens the martensite. Manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel are also used in various combinations to increase hardenability.
These steels are often subjected to post-quench tempering to improve ductility, so that extremely high strength levels can be achieved along with adequate ductility for certain forming processes like Roll Forming.
Figure 3 shows MS950/1200 compared to HSLA. Engineering and true stress-strain curves for MS steel grades are presented in Figures 4 and 5.
Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and MS 950/1200.
Figure 4: Engineering stress-strain curves for a series of MS steel grades.S-5 Sheet thicknesses: 1.8 mm to 2.0 mm.
Figure 5: True stress-strain curves for a series of MS steel grades.S-5 Sheet thicknesses: 1.8 mm to 2.0mm.
In addition to being produced directly at the steel mill, a martensitic microstructure also can be developed during the hot stamping of press hardening steels.
Examples of current production grades of martensitic steels and typical automotive applications include:
| MS 950/1200 | Cross-members, side intrusion beams, bumper beams, bumper reinforcements |
| MS 1150/1400 | Rocker outer, side intrusion beams, bumper beams, bumper reinforcements |
| MS 1250/1500 | Side intrusion beams, bumper beams, bumper reinforcements |
Some of the specifications describing uncoated cold rolled 1st Generation martensite steel (MS) are included below, with the grades typically listed in order of increasing minimum tensile strength. 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.
Martensitic steel grades provide a cold formed alternative to hot formed press hardening steels. Not all product shapes can be cold formed. For those shapes where forming at ambient temperatures is possible, design and process strategies must address the springback which comes with the high strength levels, as well as eliminate the risk of delayed fracture. The potential benefits associated with cold forming include lower energy costs, reduced carbon footprint, and improved cycle times compared with hot forming processes.
Highlighting product forms achievable in cold stamping, an automotive steel Product Applications Laboratory formed a Roof Center Reinforcement from 1.4 mm CR1200Y1470T-MS using conventional cold stamping rather than roll forming, Figure 6. Using cold stamping allows for the flexibility of considering different strategies when die processing which may result in reduced springback or incorporating part features not achievable with roll forming.
Figure 6: Roof Center Reinforcement cold stamped from CR1200Y1470T-MS martensitic steel.U-1
Cold stamping of martensitic steels is not limited to simpler shapes with gentle curvature. Shown in Figure 7 is a Center Pillar Outer, cold stamped using a tailor welded blank containing CR1200Y1470T-MS and CR320Y590T-DP as the upper and lower portion steels.U-1
Figure 7: Center Pillar Outer stamped at ambient temperature from a tailor welded blank containing 1470 MPa tensile strength martensitic steel.U-1
Another characteristic of martensitic steels is their high yield strength, which is associated with improved crash performance. In a laboratory environment, crash behavior is assessed with 3-point bending moments. A studyS-8 determined there was a correlation between sheet steel yield strength and the 3-point bending deformation of hat shaped parts. Based on a comparison of yield strength, Figure 8 shows that CR1200Y1470T-MS has similar performance to hot stamped PHS-CR1800T-MB and PHS-CR1900T-MB at the same thickness and exceeds the frequently used PHS-CR1500T-MB. For this reason, there may be the potential to reduce costs and even weight with a cold stamping approach, providing appropriate press, process, and die designs are used.
Figure 8: Effect of Yield Strength on Bending Moment. The right image shows the typical yield strength range of CR1030Y1300T-MS and CR1200Y1470T-MS as well as typical yield strength values of several Press Hardened Steels.S-8
Recent years have seen some applications typically associated with press hardening steels transition to a cold stamped martensitic steel, CR1200Y1470T-MS. One such example is found in the third-generation Nissan B-segment hatchback (2020 start of production), which uses 1.2 mm thick CR1200Y1470T-MS as the material for the Second Cross Member Reinforcement.K-45
Using the carbon equivalent formula Ceq=C+Si/30+Mn/20+2P+4S K-45, the newly developed martensitic grade has a carbon equivalent value of 0.28, which is lower than the 0.35 associated with the conventional PHS grade of comparable tensile strength, 22MnB5 (PHS 1500T). The lower carbon equivalent value is expected to translate into easier welding conditions. Furthermore, conventional mechanical trimming and piercing equipment and techniques work with the cold formable martensitic grade, whereas parts formed from press hardening steels typically require laser trimming or other more costly approaches. An evaluation of delayed fracture found no evidence of this failure mode.
Figure 9 highlights this reinforcement, with its placement on the cross member and in the vehicle shown in red. The varying elevation of this part, combined with a non-uniform cross section at the outermost edges, help control springback, but makes roll forming significantly more challenging if that were the cold forming approach.
Figure 9: Cold-Stamped Martensitic Steel with 1500 MPa Tensile Strength used in the Nissan B-Segment Hatchback.K-57
Unbalanced stresses in stamped parts lead to several types of shape fixability issues collectively called springback. In hat shape wall sections, shape fixing beads sometimes referred to as stake beads (see Post Stretch information at this link) mitigate sidewall curl by imparting a tensile stress state on both the top and bottom sheet surfaces and increasing the rigidity. Springback control to limit flexing down the length of longitudinally curved parts requires a different technique. Here, the root cause is the stress difference between the tensile stress at the punch top and the compressive stress at the flange at bottom dead center of the press stroke. Figure 10 presents schematics of the stress distribution when the punch is located at bottom dead center of the press stroke, and the shape fixability issue after load removal.
Figure 10: Cold-Stamped Martensitic Steel With 1500 MPa Tensile Strength Used in the Lexus NXJ-24
A patented approach known as Stress Reverse Forming™T-44 improved dimensional accuracy in the second-generation Lexus NX (2021 start of production) center roof reinforcement, cold stamped from martensitic steel, CR1200Y1470T-MS.J-24 Figure 11 shows different views of this part.
Figure 11: Left Image: Springback Differences Exist in Coils at the Low and High End of the Strength Specification; Right Image: Stress Reverse Forming™ Process Reduces Sensitivity to Springback (Images Adapted from Citation T-29)
Stress Reverse Forming™T-44 uses the principles of the Bauschinger Effect to reverse the direction of the forming stresses during a restrike forming process to achieve a final part closer to the targeted dimensions.T-29 Parts processed with this two-step approach are first over-formed to a smaller radius of curvature than the final part shape. Removing the part from the tool after this first forming step results in the stress distribution seen in the left image in Figure 10. The unique aspect of this approach comes from the second forming step where the tool shape forces the punch top into slight compression while the lower flange is put into slight tension. The tool shape used in this stage contains a slightly greater radius of curvature than the targeted part shape. As shown in Citation T-29, this process appears to be equally effective at all steel strengths.
Without effective countermeasures, springback increases with part strength. Related to this is the springback difference between coils having strength at the lowest and highest ends of the acceptable property range. This can lead to substantial differences in springback between coils completely within specification. However, after using effective countermeasures such as Stress Reverse Forming™ described in Citation T-29, springback differences between coils are minimized, which leads to increased dimensional accuracy and more consistent stamping performance. This phenomenon is shown schematically in Figure 12. Furthermore, unlike conventional stamping approaches, the amount of springback in parts made with this approach does not increase with steel strength.
Figure 12: Left Image: Springback Differences Exist in Coils at the Low and High End of the Strength Specification; Right Image: Stress Reverse Forming™ Process Reduces Sensitivity to Springback (Images Adapted from Citation T-29)