22MnB5 Archives - AHSS Guidelines

Cold Stamped or Hot Formed? Part 1

Equipment, Responsibilities, and Property Development Considerations When Deciding How A Part Gets Formed

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.

Equipment

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.

Responsibilities

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.

Property Development

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.

Effect GA-Coating Evolution PHS

Press Hardened Steels, Solid State Welding

This article summarizes a paper, entitled “Effect of GA-Coating Evolution during Press-Hardening on Fiber Laser Lap Welding Behavior of 22MnB5 Steel”, by M. H. Razmpoosh, et al.^R-4

The study investigates the effects of Fe-Zn diffusion layer on laser lap-joining behavior of galvanneal (GA) coated 22MnB5 steel, an Advanced High-Strength Steel designed for the hot forming process. The results indicate that by using higher press-hardening durations, the weld window shrinks; however, this results in a wider weld bead, and therefore promotes the load-bearing capacity of the joint.

Press-hardened 22MnB5, 2mm sheet steels were used in the present study. The details of the chemical composition and the as-received mechanical properties of the sheets are given in Table 1. The steel sheets were GA-coated with two different initial total coating weights of 100 and 140 g/m2 (Table 2).

Table 1: Chemical Composition (wt.%) and Mechanical Properties of the Experimental PHS.

Table 2: Weight and Chemical Composition of Various GA Coatings used in the Present Study.

Figure 1 demonstrates backscattered scanning electron microscopy (BS-SEM) and Electron probe microanalysis (EPMA) elemental distribution of a representative Fe-Zn DL after press-hardening at 860°C for 4-10 min and corresponding 900°C for 10 min. It has been observed that by increasing the press-hardening time the Zn-content decreases; however, at higher press-hardening temperatures (i.e., 900°C) due to extreme oxidation, the average Zn-content decreases severely.

Figure 1. BS-SEM and EPMA Results of the Press-Hardened Blanks at 860°C [(a) 4 min, (b) 7 min, (c) 10 min, and (d) 900°C for 10 min (DL)].

Figure 2 summarizes the effects of press-hardening time and temperature on the thickness of the Fe-Zn DL in two different initial coating weights of 100 and 140 g/m2. With increasing the heat-treatment time at 860°C, the thickness of overall Fe-Zn DL increases. However, specifically at 900°C and longer press-hardening times, the final Fe-Zn DL is not increasing. Moreover, it has been observed that at a constant press-hardening time-temperature, lower initial coating weight results in a lower final Fe-Zn DL thickness.

Figure 2: DL Thickness vs. Press-Hardening Times at the Experimental Temperature and Initial Coating Weights.

According to Figure 3(a), increasing the press-hardening time at a constant temperature results in wider weld beads. Hence, the fact that failure occurs within the FZ (faying surface) during lap-shear tensile tests justifies the slightly enhanced peak loads [Figure 3(b)].

Figure 3: (a) Joint Width, (b) Peak Load of Lap-Shear Tensile Test vs. Press-Hardening Time, and (c) Schematic of Fe-Zn DL and Laser Interaction.

This work concluded the following:

The initial GA-coating mainly evolves into a Fe-Zn DL [α-Fe(Zn)] and ZnO after the press-hardening. The thick α-Fe(Zn) phase holding 20-40% Zn; however, it was observed that with increasing press-hardening temperature, due to severity of oxidation Zn-content of the Fe-Zn DL decreases.
Due to higher oxidation, severity at higher press-hardening temperatures, and subsequent lower Zn-content, the sensitivity of the process window is less than 860°C.
Because of intensified Zn-plasma and laser beam interaction, by increasing the press hardening time at a constant temperature of 860°C (higher Fe-Zn DL thickness), joint width increases. This explains higher lap-shear tensile peak loads associated with the higher press-hardening times.