How to Select Steel Grade for Deep Drawing Parts

Selecting the right steel grade is the first decision in any deep drawing operation. The material must withstand significant plastic deformation without cracking, wrinkling, or exceeding the die’s forming limits. A poor choice often shows up as a torn part, a wrinkled bottom, or an overdrawn flange. This guide outlines a practical process for matching grades to part geometry and formability limits.

Prerequisites

Before choosing a grade, gather the part drawing, the forming sequence, and the supplier’s material certificate. You need the exact draw ratio, the part’s critical features, and the mechanical properties the finished component must meet. The forming sequence matters because a part drawn in two stages may tolerate a different grade than one formed in a single hit.

1. Identify the Critical Part Features

Start by locating the features that define the drawing limit. The deepest draw, the smallest inner corner, and the thinnest flange are usually the trouble points. Mark these on the part drawing. A 20 mm diameter cup with a 25 mm depth is far more demanding than a shallow tray with the same diameter.

The reason for this step is simple. The material must stretch enough to reach the die bottom without exceeding its elongation limit. If you ignore the critical features, you may pick a grade that looks acceptable in spec sheets but fails on the actual part.

2. Calculate the Maximum Draw Ratio

Measure the deepest section of the part and compare it to the blank diameter. This gives you the draw ratio. For example, a 100 mm blank forming a 40 mm cup has a 40 percent draw. The reason for calculating this is that formability is not a single number. It is a relationship between the blank size, the draw depth, and the material’s ability to stretch uniformly.

Keep the draw ratio in mind when comparing grades. A high draw ratio demands a material with high elongation and high reduction in area. A low draw ratio may allow a harder, stronger grade that is less forgiving.

3. Check the Material Data Sheet

Open the mill certificate or the supplier’s material data sheet for the candidate grade. Look for elongation at break, reduction in area, yield strength, and tensile strength. The reason for this check is that spec sheets list averages. You need the specific values for the thickness and the heat lot.

Do not rely on a general grade name alone. A “deep drawing grade” can behave differently depending on the manufacturer, the rolling schedule, and the thickness. The data sheet is your primary source for the actual formability numbers.

4. Match Elongation to the Draw Ratio

Compare the material’s elongation to your calculated draw ratio. The material’s elongation must exceed the total strain the part will experience. This includes the draw strain, the flange reduction, and any secondary forming steps. The reason for this match is that elongation is the direct measure of how much a steel sheet can stretch before it necks and tears.

If the elongation is too low, the part will tear at the die bottom or at a sharp corner. If the elongation is too high, the part may wrinkle because the material cannot be compressed efficiently in the flange.

5. Evaluate Reduction in Area

Reduction in area is the other formability number to watch. It measures how much the cross section of the sheet can be reduced during stretching. A high reduction in area means the material can thin out without cracking. The reason for this step is that elongation alone does not tell the whole story. A material can stretch a long distance but still fail if it cannot thin uniformly.

For deep drawing, both elongation and reduction in area must be sufficient. A grade with high elongation but low reduction in area may stretch but then split at the bottom. A grade with high reduction in area but low elongation may thin out but not reach the die bottom.

6. Consider Yield Strength and Stiffness

Yield strength determines how much force the press must apply. It also affects the part’s ability to hold its shape. A higher yield strength material is stiffer, which reduces springback but increases the required tonnage. The reason for this step is that the press capability is a hard limit. If the required force exceeds the press’s capacity, the part will not form correctly, regardless of the grade’s formability.

For deep drawing, you usually want a balance. You need enough strength to hold the shape but not so much that the press struggles. A lower yield strength grade is easier to draw but may require more support in the die to prevent wrinkling.

7. Select the Grade Based on Formability and Strength

Use the data from steps 3 to 6 to pick a grade. A common approach is to start with a high formability grade and adjust the thickness if the strength is too low. The reason for this approach is that formability is harder to change than thickness. You can often compensate for a soft grade with a thicker blank, but you cannot easily make a low formability grade into a high formability one.

The table below summarizes the trade-offs between common steel grades and their typical deep drawing behavior.

Grade Type	Draw Ratio Tolerance	Strength	Wrinkle Risk
High Formability	High	Low	High
Medium Formability	Medium	Medium	Medium
Low Formability	Low	High	Low

The reason for using a table is that it allows a quick visual comparison of the trade-offs. You can see that a high formability grade handles a high draw ratio but may wrinkle. A low formability grade is strong but struggles with high draw ratios.

8. Run a Trial and Check for Defects

The final step is to run a trial on the actual press. Check the part for tearing at the bottom, wrinkling in the flange, and excessive thinning. The reason for this step is that simulations and calculations are guides, not guarantees. The actual behavior depends on the die design, the lubrication, and the press speed.

If the part tears, the grade is too low formability or the draw is too deep. If the part wrinkles, the grade is too soft or the flange support is insufficient. Adjust the grade or the thickness based on the trial results.

9. Verify the Material Certificate

Before production, verify that the material certificate matches the selected grade and thickness. The reason for this step is that suppliers may substitute a similar grade without notice. A substitution can change the formability and cause part failures in production.

Check the heat lot number and the mechanical properties on the certificate. If the properties fall outside the expected range, reject the material or adjust the process.

Common Mistakes

• Picking a grade based on cost alone. A cheap grade may fail on the critical features and cause scrap.
• Ignoring the flange reduction. The flange must be compressed without wrinkling. A material with low formability may not handle the compression.
• Assuming the same grade behaves the same at all thicknesses. Thin gauges are more forgiving than thick gauges, but the formability limits still apply.
• Skipping the trial run. Calculations are useful, but the trial run confirms the behavior.
• Not checking the material certificate. A substitution can change the grade and cause failures.

Final Verification Step

After the trial run and the certificate check, perform a final verification. Measure the part’s critical dimensions and compare them to the drawing. Check the material for surface defects and confirm the thickness is within tolerance. The reason for this final step is that it catches any issues that the trial run may have missed.

If the part meets the drawing and the material is certified, you are ready for production. If not, return to step 1 and re-evaluate the grade or the part design.